If you are planning on having a small tank, you don't really need to worry too much about filtration. I agree with the internal sump. The quickest way to make a tank look ugly (in my opinion) is to fill the tank with visible equipment. It really steals some of the magic. My suspicion is that the nitrates came from feedings and your water change schedule/filtration in that tank is inadequate to rid the tank of nitrates. If you move over the rock, it shouldn't leach nitrates. As far as the filtration goes, quite a few people modify old filters and fashion them into refugiums for their tanks. I'm running a four gallon aquarium right now and one could argue that there is no filtration. So far so good. I just do frequent water changes, which isn't a pain at all considering 1 gallon is something like 40% of the tank volume. When you do water changes for your big tank, just mix a little extra. Here is an example of a mod that can be done easily and cheaply (it is quite popular). Lastly, I agree with Bang Guy 100% It is astounding what evaporation can do to salinity levels in a short time with these nano tanks. It has to do with the surface area vs volume ratio, something that certainly does not scale. An ATO is a great investment. I also agree with LEDs. That's what I use and I couldn't imagine using anything else. They run cool enough that my tank has no problems with temperature and they are bright enough that I can get all the lps I desire.
