Drilling a tank frustration

Hmmm, looks like an interesting array of tools :)
I can honestly say I have not drilled a tank. I have drilled thru land cut various plate glass to create various items though...
The actual cutting of the hole was easy. You stated the technic. If you go slow with the diamond head cutter and let the weight of the drill make the cut. You will be fine. Just make sure to constantly add water.

What ended up taking so long was the cheap O ring that the manufacturer of the overflow used. I only found out it was leaking after I had glued in the overflow. It broke taking it apart to get the O ring and I had to buy a new one and order a separate O ring for it.