I think you have a nice plan for the FW
sump .... a few refinements:
1. Get as big a sump as you can house ... it doesn't have to be glass, a see through rubbermaid tub works well & costs less. If you are considering a planted fuge, I would get at least a 20 or 30 gal
for the sump. <40 or 50 gal
is even better.> You need a fair bit of "head room" in the sump to hold the water coming down from the display in case of pump/power failure. In a small sump, this might mean 10-12" of space, and that doesn't leave much water depth for the plants. With a big sump, you might need 4-5" of head room for the same volume, so you can have a nice foot or so of water for your plants to grow in.
2. You can use the existing 5 gal
sump as your media compartment. If you have it above the new sump, you can just drill some holes at the bottom (or install a bulkhead & drain ... quieter) to let the water out. Basically, I am thinking of having the display drain into the 5 gal
, have the water percolate through the bioballs/media, then drain out by gravity into the sump.
3. you can get easy plants for the fuge (like duckweed or hornwort) ... or grow green algae. Soft green algae is just as good a nitrate sponge, and prob. easier to grow. But then, plants look better ....