The distance between the holes in the glass would be determined by the flanges on your bulkheads., usually 2-2.5" should be plenty of room., but lay it all out before you drill anything.,., Cant put the glass back together you know.,. lol
If your using 1" bulkheads I think 2 drains would be more than enough for a 20L., I use 2 on my 40 and at full siphon a 1" hole pulls a lot of water., Faster than my 750GPH pump can keep up with if its open all the way.,
The ball valve works fine for me., Although conventional wisdom will tell you, you should use a gate valve because you have way more control as the ball valve require very minute adjustments to get it right., But have fun finding a plastic gate valve without having to order it....
The teeth are just an attempt at keeping things out of there., Although you can tell from the required strainer on the lower drain that it doesnt all ways work., I had a couple fish and snails ride the elevator to my sump before I put it on there., lol.,. Ive seen plenty of overflows without teeth that Im sure work just fine., I would avoid any kind of mesh as itll just clog up with crap and now you have a higher wall on your overflow.,
One other thing I did that helped was to cut a scrap piece of acrylic to use as a lid for the box., a dab of silicone in the corner of the glass and she sits right on top nice an sturdy., If you do it right., All I can hear is a low frequency hum from my return pump and skimmer., almost zero water noise., Be sure to drain below the standing water level in your sump..
Going with a 29 gal now, probably should have updated the main post sry
The 20l just looked way to short with a wood canopy on it
I think I'm still going to keep the 3rd drain. It's not a flow issue, it's a failsafe thing. The idea is to tune the siphon slightly slower than the return pump, so just the tiniest trickle possible goes into the durso. If that durso happened to clog for any reason, it would lead to an overflow in the main tank, hence the emergency drain.
Sump is only 10 gal- all i could fit in the stand. It's a work in progress, the plan is to get the main tank plummed, fill it, then open the siphon and let it drain into the sump(which hasnt be baffle-ized yet). This way I know exactly how much spare room I will need when I finish the sump design.
The lfs is going to help me drill, they have all the bulkheads, returns etc there, so I guess I'll just get a general layout then finallize it at the shop.
Thanks again for your help