not a fish tank, but, ive drilled struts, water tanks for pastures, ive helped build a 1200g tank that was used for my stepdads cow farm, we plumbed it after we filled it with water, now let me tell you, that was NOT fun at all, cause, he didnt listen to me when i told him, but, david, you gotta drain some of the water out before we drill this hole in the middle of the side of the tank, BIGGGG mess, but, yes, ive drilled lots of tanks/barrels/whatnot, that are full of water or oil. i used to work in the hydraulic hose industry, 12 years experience, woot LOL, and have done numerous installations that required me to drill a customers bulk oil tank that was full of oil, since, they dont want it pumping out of the top of the tank like we recommend, noooo, dont listen to the experts, we want you to attach the pump to the side of the tank, yeah, now you got 100 gallons of oil all over the ground genius, cause u wouldnt listen to us