I think I understand your thinking of wanting something of a "centrepiece" fish; this is common with many who have tanks in the 20-30 gallon range. But generally it really doesn't work. "Larger" fish bring with them specific requirements. These tanks are actually more interesting with smaller fish. I have six tanks running presently, 115g, 90g, 70g, 33g, 20g and 10g, and the largest fish I own is my Bolivian Ram at just over three inches, and he is in the largest tank. I just can't imagine him in the 20g.
Gourami are best in groups. They are not "shoaling" fish in the sense of most of the characins, cyprinids, etc., but like angelfish and discus, gourami do have defined hierarchies and interactions, so generally a small group is best. And males are territorial, sometimes very much so. The species that would work in your tank are again the small ones, certainly not the Pearl which is so beautiful it really should have a 4-foot tank with a group of 5-6. The problem with Aqadvisor is that it is impossible to write all the factors into the program.
If the Danio margaritatus are really intended, I do think this species is setting the parameters. Here is an excerpt from a profile of this species I wrote a couple years ago for another site:
Compatibility/Temperament: Shy and quiet by nature, and given its minute size, it should have its own aquarium. Dominant males can be quite feisty, even nipping fins of rival males, but kept in a group of 20 or more lessens the damage. The tank should be well planted to break up the line of sight.
I think corys are OK here...but perhaps not Corydoras panda now that I think of it. This species occurs in higher mountain streams in Peru, specifically the Rio Aquas and Rio Amarillas (tributary of Rio Pachitea), Rio Ucayali system, upper Amazon basin in Peru. Water flow is somewhat faster in these mountain streams, with cooler temperatures; substrate is sand/very fine gravel. I have a group of five of these, and they "play" in the strong flow from the canister filter right where it enters the tank. D. margaritatus would not appreciate this, as they occur in boggy areas and shallow ponds. And here is one of those "factors" that makes Aqadvisor difficult.
I've probably thrown a few more wrenches into the works here, but there is a lot involved in combining different fish species in an enclosed system. This has been one of my primary areas of focus over the years. In those tanks I mentioned earlier, I have fish in say the 70g that would never work in the 115g, nor again the 90g, and not because of temperament or aggressiveness but solely because of the other fish's level of activity, or the water current which is different in each tank.
Byron.
