Tank drilling clearances?

[RonS]

I searched around and didn't see this question, but forgive me if this is already noted somewhere...

I'm about to drill the back of a 48 gallon, low-iron, rimless tank for an overflow and 3/4" pump return bulkhead. For the return, I will be about 4" from the side wall which doesn't bother me. However, I am looking for recommendations on a minimum down from the top edge that should be safe to drill the hole. The water level in the tank will be about 3/4" - 1". Is it safe to drill the tank so that the top of the hole is only 1" down from the edge of the glass? I want the return as close to the surface as possible, but I don't want to crack/chip the tank!!

I should note that I have purchased a Herco "Y" type backflow prevention valve, so siphoning really shouldn't be a problem. But I also want the best

 

[lmm1967]

1" seems a little close - but that's just my opinion.

I try to go at least 1.5 times the size of the hole away from any edge or any other hole.

I would also consider skipping the back flow valve - size your sump to be able to accommodate the amount of water that will drain from the tank.

Why would I skip the valve? Easy answer - something WILL plug it eventually.

I've had 2 tanks overflowing to 1 sump & using 1 common return pump - somehow, I had "things" (whelk snails to be exact) - get from one tank in to the other tank - via the sump. Same with crabs. How do they do it? i dunno - but they did. I've also had fish that just loved to go for a ride down the drain line and end up in the sump. Had there been a valve - they would have been stuck, rendering the valve useless and more than likely killing the fish.

 

[RonS]

Thank you - that "1.5 times" sounds very wise, and somewhat familiar. And just because the hole is lower doesn't mean that the actual outlet nozzle has so be!

Regarding the backflow... I'll think about it. Anything mechanical introduces another point of failure, and I'm always hesitant about that. That valve is very highly regarded, but my sump will be huge compared to my tank volume, so there would have to be a lot of siphoning going on.

Thanks again!

--Ron