What size hole to drill in tank

I wouldn't use anything that can wipe out your entire system. If antifreeze isn't safe to dump down our sewer drains, would we want to risk it getting in our tank? Residue can linger on the tank wall and silicone when the drill bit breaks though and contaminate your tank. Plain old water is the safest way to go.

 

I just clamp a hose onto the tank and let the water run continuously at a very slow rate. (small steady stream) This works really well for me and you don't have to stop to cool things off.

 

I use pet safe anti freeze (I can here people still cringing.. lol) and I just put up a clown breeder station about 30 tanks and I just put a towel under where I drilled to catch the pet safe anti freeze. I understand there are still uncertainties that some people have but I assure you I have done 4 set-ups like this and had no effects from it. I will post pictures soon s my batteries recharge.