I actually have two sets of fans I bought, one is 3" which I was going
to use, the other 4 1/2". I think I'll probably install one fan on
each side of the canopy. The 4 1/2" fans will be a little harder to
install as well as be louder so if I can get by with 3" I'd like it.

Once again, this is a 125 gallon tank. Should I go with the 4 1/2" or
is 3" enough? I know I could try 3" and then later make the holes
larger but I'd rather do this only once.

As well, I know the evaporation is important to cooling but how much
will installing these fans increase evaporation? I tend to add around
5 gallons of water to my fishtank weekly.

Also, I have looked at some pictures and it appears I simply need to
drill a hole where the fan will be installed, 3 to 4 1/2 inches. What
type of tool does this best?

Thanks again for your help.