Tempered glass cannot be drilled, so if your tank bottom is drilled, either it was done before it was tempered (very likely) or it isn't tempered glass despite what the label says. Typically all shaping, sizing, and drilling is done before the final product is tempered.
Theoretically, I think You can anneal the glass and drill it, then re-temper it, but I've never seen or heard of anyone actually doing that.
Every once in a while, someone claims that they've succeeded in drilling or cutting tempered glass, but it seems a diminishingly small number of claims compared to the huge numbers of failures.
Either way, if it IS drilled, and didn't shatter in the process, you should be fine.
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