Ok, now for the stupid questions.
I am getting ready to set up my 50 gal tank and I have never done a large tank.
Do I consider it a 70 gal because of the 20 gal sump?
I need to know how many watts the heater needs to be.
How strong does the pump or power head have to be to return the water to the tank from the sump? The sump will be on a shelf 2 feet under the big tank.
I also need to know if there are any other issues that I may encounter with this project.

You don't really need a sump setup for a 50 gal. Canisters will do just fine. One power head in the corner to move the water. You'll save money using canister.

I was planning on using part of the sump as a mini grow out tank for fry, and I figured that the extra 20 gal would let me stock the tank heavier. :laugh:

What are you going to put in it?

I think I've finally made up my mind :laugh: I've been thinking about this for a few months, like since August :laugh: .

I want
12 Endlers Livebearers
2 Clown loaches
3 Khuli loaches
6 Apistogramma cacatuoides

And possibly a small shark and pleco later.

If it is all rigged for a sump then I would use it as such. You should still be able to use the sump as a grow up tank. You calculate the heater by teh total water volume. if you are in a cold weather climate I would consider two 150 watt heaters and I would put them in the sump so as to reduce clutter.

The water turn over of the sump will, in part, be a limiting factor on the amount of fish. A wet dry sump is just about the most efficient filtration possible. Might not hurt for cich1 to add his two cents here either.

It's not drilled but I think I'm going to just use a power head to move the water from tank to sump and back. I was figuring on putting the heaters in the sump along with anything else I need but don't want seen.

as far as the return pump is concerned i think 3 to 4 times the volume of the tank is the minimum. you'll need to use a pump designed to pump water up into the display from the sump. powerheads are designed to run with no "head". depending on the distance from the sump up to the display(this is your "head" distance) you will probably want a pump that will at least have a max head of 8' to 10'. make sure that the overflow box you use can handle the pressure the return pump is operating at. if you have a pump running at say 350 gph you would want an overflow box that could handle at least that much most likely more flow just to be on the safe side.

Sounds like I need to do a little more research on this sump, Hubby says that we are going to use a bilge type pump. Time to revisit those links cich1 gave me :hmm:

So, how do I calculate this?

are you going to build your own overflow box? if so then i guess the old trial and error method is what i would use(outside), but glaive could probably help you out with a formula to actually calculate it. if you were going to buy one then that makes it easy, they are flow rated so you would just need to buy the appropriate sized overflow.here is an idea of what you'll be looking at to buy one, overflow boxes.

here's another link for utube overflows overflows

I was going to build it myself. I guess that I'll have to see if I can get some measurements for the box sizes from some sites and figure it out.