Green Spotted Puffers require full marine conditions and Figure 8s are brackish. I believe Archer fish are also brackish. Baby whales aren't good with many other types of fish as the electrical pulses they use (like sonar) make a lot of other fish wig out. And trust me, you will never see it, either.

Its noctural and rarely comes out of its cave or the plants, and it does need both.
I love my pitbull plecos, which are pretty unusual (need at least 2-3 minimum as they like the company of their own kind). Kuhli loaches, while not all that unusual, are loads of fun to watch and should be kept in groups of 6 or more.