If I understand all Im readin, if the power goes out, or the return pump stops the tank will only drain so far if I plumb it correct? - yep, why it is better to have the drains or the overflows at the top of the tank rather than at the bottom.
But the sump must hold the normal volume plus the extra that will drain down right? - correct. How do you tell what the correct water level needs to be? When you fill it don't have the pumps running. Get the tank filled to the overflow line, put enough water in to fill the sump how you want it to be at its maximum. Turn on the pump, the sump level should go down as the water enters the drains. When it stops it goes back to the way you had it. The taller the sump, (still have to cover the pump intake at the low point), the better really, and the bigger the better so you don't have to be to exact with water levels.
I see you can buy a kit of sorts for the overflows that supposidly has all you need in it. I wonder if its cheaper to buy the pieces at lowes or the kit? Always cheaper to buy at lowes, but I would check the tractor supply co. near you if you can for a cheap utility setup. If you want something nice like krish has, you might be better off buying it in the classifieds here. They can be expensive at a local store, and the parts are pretty expensive, thick acrylic isn't cheap. Some of the other sponsors could probably hook you up.
Can you plug the holes and do the sump/overflow later for cost reasons?
yeah, but once you have the pump and it is setup it is done. Once it is setup there is not additional cost really with a refugium except a 20w light. I think they cost around $15 a year to run in electricity. You would need a pump anyway in the tank. You can get those cheap at a place like lowes, they have submersible pond pumps. Got a friend that uses them and they work fine.
