I have my rts in with pearl gouramis, neon tetras, white clouds, yo-yos, and kribs. I had two angels and a b. ram but I moved them to their own tank now, and he doesn't bother anybody ( now in a 55 ), but when I had him in 30gal., with the gouramis, white clouds, and neons, he was fine until he reached about 4" then he started to torement the gouramis pretty bad. Then I added 2 weather louches which he absolutely hated. everytime they came out from under a rock he would nail them. Hit them so hard you could hear it from the other side of the room. Needless to say I returned the louches.
Moved them all over to a 55 and added the aboved mentioned fish, oh yah there is a rubber pelco in there to and some ottos i forgot about and all is fine. IMO, I would draw the conclusion that even a 30 is a bit small. I find rts tend to get bored and thus chase fish. I would go into greater detail to explain why I think this, but my post is already long winded enough. Mine loves to play in my power head. I think a general rule of thumb with a rt the longer the tank the better or lots of fast current. Just my opion though. They are super cool fish mine is just over 6" from head to tail and would recommend one to anyone, but I do think it would require a little mor space than a 20 gal, would give. niner