I wouldn't make money, but I could probably recoup the money I put into it. I'm not sure I like the 3/8" thickness, but I've seen quite a few commercial tanks built that way...so I wonder if some of the hype on that is over cautious... I don't know, just wonder. Of course, if it were thicker, I might not have this issue...
This is really just basic engineering. A 90 degree inside corner is the start of a crack.... think about a piece of paper cut in an L. Pull the legs... where is it going to break? At the inside corner.
Acrylic is the same way basically. It creates a weak point. A radiused corner can handle substantially more load.
The rule of thumb thrown out around here is a 2 inch radius.... which is technically a 4" circle. I dunno, I used a 3 inch circle saw to make my corners and I think they'll be just fine. I'm planning to reinforce the center brace and end where the external overflow is with a 2"x1/2" by tank width crossmember mounted above the tank.
Again, basic engineering applies, what I want to do is resist bowing in the top. The reason is that when the sides flex, they will pull or push (distort) the top. If all that you have for the top is the center brace it's going to flex. So if you reinforce it with a perpendicular member wide enough to resist shear forces it will resist the flexing of the center brace and thus the side walls. The beautiful thing about acrylic as opposed to glass is that you can actually fabricate and weld like you would with steel. As a result it behaves basically the same way.
While the forces at work seem powerful and mysterious.... rest assured 4,000 pounds of water spread over the sides of a tank isn't really that much force.... Your car tire hitting a bump at 30 mph has substantially more going on to put it in perspective.
so back to your tank.... WeldOn is a solvent. You will be able to see if WO3 seeps in there or not.
the best way to remember where to patch is to think about a bicycle tire. Would you patch the outside? Probably not. In fact if you could.. you would turn the tub inside out and patch the inside of the tube because that is the strongest place to put a patch. The air wanting to escape (or water in our case) would have to push the patch out.
Quick and dirty? Get some 1/4" cell cast scrap from the local plastic dealer. It's usually $1/pound around here. Cut a piece that is 2.5" larger then the outline of the damaged area.
Get some WO16 and WO3 along with an applicator syringe and some needles while you are there. Use the syringe to carefully apply solvent to the inside of the crack. Make sure you only apply solvent to a horizontal surface from the top side. It will run all over creation and make things look sloppy if you disregard this.
Let it dry for a couple of hours. Flip the tank over so the outside of the crack is facing up. Repeat the solvent seep process. A little goes a long way, but it's alright to let it puddle. It will evaporate if it doesn't soak in.
Let it sit for a few hours. After 2 hours the solvent weld has a strenght of 800 PSI per IPS' website. After something like 8 hours its at 1500 psi and after a week it's at something like 2000PSI. Acrylic has a tensile strenght of approximately 10,000 psi I seem to recall.
Now, if you want to patch it, take your patch and carefully and evenly apply wo16 to it. Apply some to the inside area where you are going to put it. The goal is to get it even so you don't make ugly air bubbles. Carefully apply it and push down. I recommend putting some weight here to compress the air bubbles out. The problem with WO16, IMHO is that it's a thicker material... It reminds me an awful lot of Testers Model Glue actually. Anyhow it's just prone to trap air bubbles in a application like this. They aren't harmful, just ugly.
The purpose of the overlap is to create a structural plate that bypasses the damaged area. Any water pressure pushing out will be retained by the 1/4" material and the remains of the tank, which I think are probably fine, but damaged visually.
If you want to be cheap you can use the material from LOWEs or Home Depot... however, it's extruded. Extruded Acrylic is chemically different and prone to misbehave. It's alot less stable, it doesn't machine, cut or react to stress well. Another member on here, Acrylics, has explained it in depth. So just realize that yes you can use HD acrylic... not polycarbonate or lexan that's like brass and aluminum when you are working with steel. They don't weld together using normal processes. In fact, metal is a great analogy. Brass is like Lexan, Aluminum is like Polycarbonate, Extruded is like Steel, and Acrylic is like stainless... they are all metal, but have different properties and strengths. Like stainless, Acrylic is the more particular to weld and can be demanding. Don't let it stop you though....
The worst that will happen is it will look messy.....
Hence I think you just found a great spot to put a powerhead.
I personally would WO3 it from the inside and out and then water test it. Give it a few days. If you don't have leaks, then get your kids to splash around in the tank with their hands. This will simulate wave stresses.
If you find you have a seep, you could try to plug with WO16 as well.
I doubt you'll have any issues. What it appears you have are stress fractures where there was a point impact and it fractured the material. A harder hit would have knocked pieces loose.
Of course, I'm just a guy in Texas who likes coral reefs..... I am not an engineer, I don't do this for a living, I didn't stay at a Holiday Inn Express and for all you know I weigh 400 pounds, drink 12 cans of coke a day, and love twinkies and post on here in my underwear. (2 of those statements are true and to give you a hint I'm not that heavy and I wear clothes at the computer.
)
I do however like to understand what I'm doing, separate the BS from the fact, and play it slightly conservative with my engineering type stuff. That said I'd still solvent it to get rid of the cracks and then see what happens. My guess is nothing will happen and it will be just fine.
