i havent drilled my tank yet. everytime i get ready to place the order for the bits, or go out to get sump materials, something always comes up. i've done a ton of research on it though, because i dont want to ruin my 125g. there have been a few threads on here that entail amateur aquarists with little or no glass drilling experience, successfully drilling their setups. king_neptune comes to mind. i wouldnt drill it partially filled personally. i'd be worried about glass dust getting into the water. it can be done while the tank is upright, but keeping the bit wet to keep it cool can be difficult.