Did I miss something? Why are you talking about building a sump now? Those are generally only necessary in tanks with very high bioloads and not enough filtration on the main tank. I don't think you'll need one for the tank you have planned.
There are two things to do when ammonia is formed by the degradation of proteins in the food you feed your fish.
1. Feed the ammonia to beneficial bacteria which live in your filter media and then remove the nitrate they excrete with regular water changes.
2. Feed the ammonium to plants.
A sump is usually employed when either neither are possible or neither are sufficient. Here are two examples.
1. In a marine tank where the fishkeeper doesn't like macroalgae, a refugium in the stand under the main tank is an easy way to get the benefits of the macroalgae without having to look at it in the main tank. Marine fish are very sensitive to nitrates because the ocean is large enough to dilute any sudden concentration spikes, so having a filter alone could lead to nitrate spikes that would kill the very expensive fish. The macroalgae refugium/sump is a good way to keep nitrate down if a perturbation in the levels occurs in between water changes.
2. An African rift lake cichlid tank, where again the large water volume made nitrate spikes not a thing the fish ever had to deal with for thousands and thousands of years. Bacteria filtration alone is often not a good solution for cichlids because they're so extremely sensitive to nitrates, and a spike in between water changes can kill them. I used to have Neolamprologus multifasciatus but one day my nitrate bumped up a little higher than it was normally and they all died... Cichlid tanks are often heavily stocked to decrease aggression. This means there is a high bioload with lots of nitrogen being degraded into ammonia. And worse, the cichlids rip apart any plants. So a refugium is a good idea for cichlids (plants don't survive long in the main tank).
But for your fish? Why are you building a refugium/sump? You can put stream plants in your main tank without the fish intentionally ripping them apart, and more importantly, you can have a nitrosomonas/nitrospira bacterial filter. Our native species of fish are much much less nitrogen sensitive than those from the African rift lakes. There's no reason not to have a normal filter on the tank and to do regular water changes. If the nitrates rises up to 30 ppm over the course of the week between water changes the fish won't care. Stream and pond fish are cool like that; they've evolved to be less sensitive to nitrogen spikes, 'cause they had to be.
Edited by EricaWieser, 22 March 2012 - 01:44 PM.