Weldon works wonders and is what almost all acrylic tanks are assembled with in the first place. You can use #3 or #4 with an acrylic patch over the hole. The solvent is water thin so it wicks or capilates between the two pieces. Cut a piece of acrylic a little larger than the hole, place it over the hole and using a needle applicator simply squirt some weldon all the way around the piece. Let it sit for 30-60 seconds so the solvent can soften the acrylic a bit and then put some weight on it. Not too much though. You just want to press the pieces firmly together and leave it for a few hours. You can also use weldon #16. It's more like a model glue consistency. This you would just apply the cement/solvent to either the patch or around the hole on the bottom of the tank and then set the patch over the hole, again giving it a little weight to press the two together. Should be good to go once it dries. HTH