For anyone needing to drill a lot of tanks, I found this hole saw guide to work very well and much less of a hassle than the guides I made.

https://www.amazon.com/Adjustable-Hole-Saw-Guide/dp/B000NB72P2/ref=sr_1_3?dchild...

There are much cheaper ones but they have very small suction cups.

That guide was used in conjunction with a water swivel similar to the one linked below.  However, mine has a drill chuck instead of a collet and it has a small valve instead of just the nipple. 

https://www.amazon.com/Almencla-Straight-Ceramic-Drilling-Watering/dp/B07P2JQRTP

The hole saws I use are the bonded diamond style where the diamond is dispersed through out the bonding material and is good for a 100 tanks or more (I have drilled hundreds of holes with mine).  I know Jehmco carries them among others.

Even thin glass 2 1/2 gallon tanks drilled successfully with the setup mentioned.

Ken