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Project: Frag and Refugium Tanks


In addition to the display tank and sump, I wanted to include a frag tank and a refugium in my system. For those who may not be familiar with the terminology, a frag tank is used as a convenient place to grow out SPS frags, Zoo colonies, etc. The refugium is a place where macro algae, copepods, and all manner of creatures can be raised without danger of predation by the fish and invertebrates in the main (display) tank.

I decided to use cast acrylic sheet to build both tanks. The two tanks have the same sized footprint - 48" X 24". The frag tank is 12" high and hold 50gals, while the refugium is 16" high and holds 75gals. The tanks will be connected to the main system via Sched-40 PVC pipes (1-1/2" feed and 2" return), which will be run from the frag/fuge tanks, along the back wall of the fish room, and then to the sump.

As material for building the tanks, I purchased two sheets of 3/8" cast acrylic. I could have done with less but I want to use one solid piece to create the tops/euro-bracing, etc. I've learned not to try to "cheap out" and use extruded acrylic for tanks. It's very difficult to cut, rout, and form without melting and it just doesn't have the strength of cast material. The material for the overflow boxes will be 3/8" cast black acrylic. Didn't have to buy any - I've got lots of scrape from a project a couple of years ago. What follows is an overview of the process I use to build acrylic tanks.

My tanks are all built in the following order:

  1. weld the two ends to one side - ends are on top of side piece
  2. turn over and place on top of other side piece - weld in place
  3. place side and end construction on top of the top piece - weld in place
  4. turn over and place on the bottom piece - weld in place

The Tools, Methods, and Materials

I used the the pins method for welding and Weld-On 4 solvent. Weld-On 4 has the consistency of water and I find it pretty easy to use. You just have to be careful not to splash it around - it leaves nasty marks.

To deliver the solvent I use a squeeze bottle with #25 gauge needle:

For pins, I've started using #18 gage acupuncture needles, but I think they are a little too thin for my liking, so I'll probably go back to baggy ties with the paper stripped off:

So, as far as the materials needed to do the actual welding - it's very little. I use a small plastic syringe to get the solvent from the can into the squeeze bottle. That makes the following everything I need:

Some of the tools I use when building an acrylic tank include spring clamps. These are great for holding pieces of acrylic together while they are being welded:

Weights. Whenever acrylic parts are being welded, pressure is necessary to ensure a perfect joint. Weight can be used to apply that pressure. I've just taken normal building bricks and covered them with duct tape to make portable weights. Works great!

Other than a table saw, a router is the most important tool when working with flat acrylic. This is a 2-1/4HP model. It has plenty of power but is light enough to be handled easily:

Whenever two pieces of acrylic need to be welded at a 90 degree angle, this homemade jig works great. The spring clamps hold the acrylic part tightly to the jig:

I often have to include overflow weirs in a project:

Both of the tanks in the current project will need one. There are a number of ways to make them. Most ways have to do with a makeshift jig of some type. This can be difficult, and at times, dangerous. I own a woodworking tool from Leigh called a dovetail jig. The Leigh model is very convenient because you can add accessories to accomplish different jobs. One of the accessories I have is for creating finger joints. Finger joints are often used to hold the sides together when making boxes and drawers. The thing about 1/2 of a box joint is - it is exactly like the fingers we need to make a weir comb. This is the Leigh jig with the finger joint accessory installed:

Here's a close up of the finger joint jig. You can see the piece of black acrylic stock with a number of fingers already cut. The protective paper is frayed where the cut was made, but the acrylic is cut very cleanly:

Together with a router, the jig makes perfect weir combs, every time. A two edge straight bit is inserted in the router. Notice that the bit is in a small collar. A piece of acrylic is placed in the jig, under finger joint accessory. The extension on that collar follows the finger extensions on the jig.

What comes out is a perfect weir comb:

I cut all the sheet acrylic used for the tanks on a table saw. That leaves a decent edge:

But not good enough for what we need. Using the pin method of solvent welding requires a very smooth edge so that the solvent wicks easily and evenly into the space between the pieces being welded. So, the edge has to be smoothed. There are two tools that can be used for the smoothing: a jointer or a router. A jointer works well, but if you aren't skilled in it's use, you can end up with work pieces that aren't square. I prefer to use a router. Almost foolproof, and leaves a perfect edge:

When finishing the edge, I use the router to shave a very thin layer of material. I made a special setup for this on my table saw. I drilled a hole in the extension table to the right of the blade and mounted a router below the table so that the router bit could protrude through the hole:

That is a double bladed straight bit - a big one. It has a 1-1/2" diameter. You don't really need that big a bit - a normal 3/4" diameter bit would do just fine. I like the bigger bit because it makes absolutely perfect cuts.

If you use such a large bit, remember to use a variable speed router so that you can turn down the speed to about 10,000 RPM! Running at a faster speed could very well burn and melt the acrylic. More importantly, large bits turning at more than 10K revs. just aren't safe!

To make the cut, I make use of the table saw's built in rip fence. Normally you would use the left edge of the fence when cutting stock with the saw. When edging with the router, I use the right edge. I move the fence until the right edge is the correct distance from the router bit. That means, so that the router bit will be shaving just a little from the work piece - less than 1/32". That looks like this:

WARNING and DISCLAIMER - at this point, I should explain that you should never use a fixed router with the work piece placed between the bit and the fence. This situation lends itself to the work piece being caught by the bit, forced into the fence, and thrown off the table at very high velocity! A dangerous situation - so don't do what I'm describing here. I do it because I've had lots of experience, I take every precaution so that nothing bad happens, and it's the only way to get really perfectly parallel edges.

Once a piece has been completed, I remove a strip of the protective paper that covers the acrylic sheet. This allows viewing the edges that are being welded and keeps the paper from interfering with the welding process. I normally just use a straight edge for this:

Then, after removal of the paper:

Once the paper has been removed, assembly of the tank can begin.

Glue-up Using the PINS Method

So now, we can do some welding (gluing). I always start a tank by welding the ends to one side. Here you can see I've placed an end piece on top of a side. It's being held in place (and square) by a home made jig and spring clamps:

Here's a close up of the joint. Note that the jig has a corner removed at a 45deg angle. This keeps the jig from interfering with the solvent. You can see that the side piece - on the bottom, in the pic - extends about 1/8" past the end piece. This provides a lip or shelf, on which the solvent needle can be rested when squirting solvent into the joint:

These strange looking things are furniture/cabinet shims. I use the plastic ones because they don't swell if they get wet. You can find them at most big box stores:

Here you can see that I've placed shims under the bottom piece of acrylic (the shims are approx. 6" apart) and a needle in the joint above each shim. The point here is that we want the space between the two pieces that was created by the needles to be the same height along the entire joint. A crooked work surface, etc. can cause problems here. First the shims are inserted only about 1/8" under the bottom work piece. Then the needles are inserted into the joint about 3/4 the thickness of the acrylic. The shims are then adjusted until each needle is being held in the joint by approx. the same amount of pressure. Inserting a shim further increases the pressure, backing it out lessens the pressure.

Once this procedure is complete we can get to the fun (and hectic) part. The welding. As mentioned earlier, I use Weld-On #4 solvent. It has a watery consistence and dries fairly quickly, which means you have to move right along when you are working with it. I take the solvent bottle (that's been filled 3/4 full), place the tip of the needle at the beginning of the joint and rest it on the little ledge created by the overlap of the bottom work piece - then I carefully squeeze the bottle and watch for the solvent to begin emerging.

Once I see the solvent, I draw the needle along the joint. I try to put a constant and even squeezing pressure on the bottle. You can't really see it in the pic, but as you're moving the bottle along the joint, you can see the solvent being drawn into the joint due to capillary action. Watching this, I can make sure that entire joint contains solvent and ensure no voids exist. Once I have completed a joint, I wait until 30 - at the most 40 - seconds have passed, then I begin removing the needles.

If the project you are working on has more than, let's say, a 48" length and 24" depth, it's better to have two people on hand. Otherwise it becomes difficult to ensure that the needles are not in place more than 40 seconds before they are removed. With two people, one can concentrate on applying the solvent and the other on removing the needles when the correct time has elapsed.

I make sure to steady the vertical work piece a little with my left hand while I pull the needle out with my right. This helps prevent the work piece from slipping due to the force of the needle being removed. As soon as the needles have been removed, I check for bubbles (voids) in the joint. If I find one I try to remove it by increasing the pressure at that point by pushing the closest shim in just a little. This increase in pressure will often cause bubbles to disappear. When that step is complete, I place weights (bricks wrapped in duct tape) on top of the vertical work piece. This helps press the joint together and stops voids from forming while the solvent dries.

Here you can see the beginnings of both the frag tank and fuge. The farthest one has a temporary divider and top piece added to support the weights.

Here is a closer view. Both ends have been welded in the pic:

Both my frag tank and refugium will have an internal overflow in the front right corner. I chose the front corner for ease of access. I'm not really too worried about how the tanks look - ease of maintenance, etc. is more important. Each overflow box consists of one side and one end, each has had teeth routed in the top to form the weir. Here you can see the overflow box being solvent welded into the refugium:

From a different angle:

From the other side - showing the overflow weir:

I first welded the two interior (black) pieces of the OF box together and then let them dry. Then I placed the completed assembly in the tank and welded the part facing down to the side piece. When solvent welding, you can only weld horizontal joints. As mentioned earlier, the solvent is very watery - trying to weld a vertical joint will result in the solvent merely running down the joint. I used blue painters tape to hold everything in place. Once that dried, I turned the tank on it's end and welded the remaining side of the box to the tank. This pic shows a judicious use of weight to ensure the (not so perfectly square) box makes full contact with the side of the tank while it's being welded:

Next, the entire assembly is placed on the acrylic sheet for the bottom. The bottom piece is 1/8" longer and wider than the actual size of the tank. This creates the ledge along which the solvent needle will be rested. The shims and needles are placed around the entire tank using the method already shown. When welding the bottom and top pieces, it's a good idea to have a helper. Any time you're building a tank that is over approx. 24" long, the time it takes to inject solvent all the way around the bottom/top is longer than we want to allow a needle to remain in the solvent. If you have a helper, he/she can begin removing needles 30-45 seconds after the solvent has been injected at their position. If you're wondering why I have not posted a pic of this process - I forgot to take one.

Before I attached the bottom of the tanks, I drilled two 2-3/8" holes - the correct size for the 1-1/2" bulkheads I'll be using for the overflow. I used a regular hole saw. I purchased this set at Home Depot:

When cutting the holes, using a hole saw, ensure the work piece is laying on a flat support and allow the weight of the drill to do most of the work. Applying too much pressure can easily damage your acrylic.

I'll be building my version of a Hofer Gurgle Buster to use in the overflow boxes of both the frag tank and refugium. My setup calls for two 1-1/2" standpipes in each box - never can have too many standpipes. Together, the Frag and Fuge will be fed by a Gold Reeflo Dart. I intend to dial it back a little, to about 2000gph. At 500gph per standpipe, I'm confident that nothing can go wrong.

Completing The Euro-Bracing

Both the frag tank and the refugium will be Euro-braced with an integrated middle brace. I've chosen to go with 3" bracing along the sides and ends and 6" for the middle. There are two different ways to proceed here. It's possible to just cut 3" wide strips for the bracing, cut them to the lengths required an then weld them to the top of the tank. This does work, and if done carefully, is sufficiently strong, and material used is minimal.  I prefer using one piece of material which has been cut the size of the top.  I've found that the easiest way to complete the top of my tanks is to rout the acrylic sheet before it's attached to the tank. I do this using a router with a 1/8" flush trim bit:

The flush trim bit is designed to follow a template which has been attached to the work piece. Rather than construct a full sized template, as is often suggested, I always piece mine together - saves a whole bunch of time and trouble. Here are the pieces for the template which will create the Euro-bracing for both tanks. The pic shows 3" wide (the width of the bracing) strips of 3/4" MDF. The strips have been cut to a length that will form a rectangle with sides that match the dimensions of exactly half the size of the top of the tank. You'll see what the small triangular pieces are for in a moment:

I use double sided tape to attach the pieces of the template to the top of the tank:

Once the tape has been attached to the MDF template:

We can start applying the pieces to the tank top:

And here it is, finished and ready to rout:

The rectangle's outer edge is aligned with the outer edge of the acrylic piece. Here, you can see that the small triangular pieces have been attached at each inner corner. They serve to enlarge (strengthen) the corners of the bracing. You most often see this done using a quarter circle, but straight lines are much easier to cut.

What is shown in the pic is a template that covers exactly one half the top of the tank. The router and flush trim bit will be used to remove the material inside the rectangle. The ball bearing at the end of the flush trim bit will ride along the edge of the MDF template, guiding the cutter through the acrylic sheet.

To do this, a hole must first be drilled through the acrylic to allow the bit to pass through. I usually use a spade bit to do this. The hole is drilled close - but not touching - the inside edge of the template:

When the hole is done, the top is turned over. Here you can see the hole just drilled. The template is now resting on the work table:

The router is placed on top of the acrylic sheet, with the bit going through the hole. The bit has to be adjusted so that when the router rests flat on the acrylic, the bearing (under the sheet) rests against the template, but is low enough so that the cutter will cut completely through the sheet (in this case 3/8"):

Now it's just a matter of routing along the edge of the template.

Make sure to follow the rule: If routing around the inside of a template, rout in a clock-wise direction - if routing around the outside of a template, rout in a counter-clock-wise direction. This ensures that you don't do what is called a climb cut - where the bit is turning in a direction that is with the cut, instead of against it. Climb cuts are very dangerous as the router can easily get away from you. This situation is almost guaranteed to ruin your work piece.

Here's the completed cut. You can see that the template has helped create a nice Euro-brace pattern:

To continue with the top, I will remove the template pieces, make whatever changes are necessary to handle the overflow section and then apply them in exactly the same manner to the other end of the top piece and rout again. This way we have 3" Euro-bracing with a 6" (doubled up 3" template) center brace.

Unfortunately, I accidently erased the memory chip with the pics of the final stages of completion for the tanks. All that was left to do was weld on the top pieces, and rout all edges to clean up the overhang left from the welding process. A 1/2" flush trim bit is used to do that. I also use a 3/8" quarter round-over bit to clean up the top edges - makes it look nice.

Here are some pics of the frag tank being leak tested. You will notice two things that don't jive (is that word still used, these days?) with my previous posts.

1. The top consists of two pieces instead of one, as I had described earlier. The reason is, I found that I messed up and measured the top incorrectly (1/2" too short - mistakes happen). I didn't want to use another 4' X 8' sheet, so I made do by cutting the too short piece in half. When I glued the two-piece top on, there was a 1/2" gap. Shouldn't have any influence on the strength of the tank.

2. The bracing over the right hand half of the tank is not a mirror image of the left side. I changed the plan and routed a separate opening for the overflow.


Todd W wrote...
Man what a nice job you did here. I am looking for some frag tanks....wanting several large ones....whats the cost on doing this?
Glass Reef replied...
Sorry, Todd. I don't do builds for hire. Why don't you consider a DIT project? It's easy.
25th September 2012 10:52pm
JULES wrote...
Great description and pics - very clear - , so these are 25 gallons each ? Love the 90 degree wood jig - simple idea.
Do you think my local cabinent shop has a dove tail jig ?
15th September 2012 10:01am
Jerry Barnes wrote...
Nice article. Everytime I do arcylic its the template piece that drives me nuts. Never thought of it in simple terms like you did. Never thought of double stick tape holding the template either. I'm always trying to clamp it and avoid interference with the router base. Great ideas you have. Thanks for taking the time to write it up.
Glass Reef replied...
Glad you found the page worthwhile, Jerry. Double sided tape is the best thing since sliced bread - couldn't get along without it.
19th January 2011 12:50pm
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