I bought a diamond bit from Jehmco, because I figured in the end I'd be drilling all tanks, didn't skimp on price, and have drilled maybe 8+ tanks with it so far.
Has taken no more than 2 minutes for the thickest of holes on 100+ gal tanks, using a simple electric hand drill, and I drill the side with the tank standing in place.
Has taken no more than 2 minutes for the thickest of holes on 100+ gal tanks, using a simple electric hand drill, and I drill the side with the tank standing in place.