i wouldnt worry much about cracking it, that fiberglass is pretty strong. Ive blown through fiberglass tanks with 3.5" bits before to run 2" pipe. to be overly cautious u can lower ur water level, clamp a piece of ply on the inside as a backer and drill from the outside. Look for a drill bit with multiple small teeth also. The more teeth the better. Id prob still drill on each side tho so it comes out clean/to prevent the epoxy paint from chipping. Ur guide bit in the center of the hole saw will keep u lined up on both sides. (ime chipping always occurs as u go through the opposite side so ive started doing half n half each way)..Nerve racking to drill a brand new tank i know... u should b more than fine tho. I also like using battery ppwered drills, the corded type are too powerful and are alot more prone to snagging/chipping. too much torque. Be patient while u drill and confident! ull b happy when its over lol...
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Not sure if u have a "handyman"/true vale/ or ace hardware in ur area... they have these smaller tooth bits opposed to homedepot/lowes who tend to stock the larger/less toothy ones.
