I have one Rainbow in a well planted large tank with lots of roots, driftwood and slate rock caves to suit his preferences. He's made his most frequent home in one of the caves near the back corner of the tank and has become somewhat territorial but is still quite tolerant of the Long-fin Danios, Oto Cats, and Cory Cats that share his habitat. The exception is that he will chase the hyperactive Danios from "his" particular cave while allowing the Bronze and Julii Corys to hang out there. He feeds best on the tank's natural algae and a frozen seafood mix while seldom partaking of the algae wafers that I throw in in small pieces daily. I am concerned that with the Otos' appetites there will consistently be too little algae for the Rainbow. However, it s still way too early to base many conclusions as to his behavior and his responsiveness to foods since he is still quite young and I've only observed him for about two weeks now. He is a beautiful and striking fish, dark grey - bright orange-red finnage, making infrequent rounds through the tank, swimming gracefully with his fins always extended. He seems quite happy with his environment. I'm anxious to observe his growth and habits as the tank continues to mature.
I have noticed that when the Rainbow Sharks get excited their fins tend to lose some of their color and become more transparent. When there's nothing going on and they're mellowed out, their fins are more brilliant.
I've had a redfinned shark for over 2 months. It spends most of its time under a piece of driftwood. It comes out from time to time for a quick swim around the tank, and then back to its spot. It seems very tolerable of cory cats, perfectly willing to share its driftwood shade. Initially it chased the tetras I had away. Even if they where just above the driftwood. However, it would never chase any one fish for a very long time. Maybe there where just too many to chase. It never actually nipped another fish. During the second month this chasing has practically stopped altogether. It probably feels it has secured its territory. I also have a large opaline Gourami that it chases briefly from time to time. The Gourami doesn't seem bothered by it. Most of the aggression from the redfinned shark seems to occur during feedings.
I have Rainbow Sharks also. After reading comments on this page I feel I need to add a few comments. The first is that these fish seem to have varied personalities - much like humans! My 2 rainbows are very playful and surprisingly not very territorial. They share the tank very nicely with bala sharks, angelfish, tiger and cherry barbs, and some kind of a shark which I am not sure of. As with all animals, each fish has special needs. As a fish owner you should be ready for these needs to change. My last comment is that I bought these rainbows to use in a 10 gallon tank for my kids. It worked good as long as I had fewer than 6 fish in the tank. I have now put them in a 55 gallon tank where they are working better with larger numbers of fish.
I have a VERY ACTIVE redfinned shark that doesn't harass the other community fish - he kills them. So far, he has killed a betta, three zebra danios, a tetra, and his latest victim - a tiger barb that was bullying all the fish in a buddy's tank that was put in my tank. At first, he would just nip at fins. He then started eating the fins so badly that when I would check the tank the next day, the victim fish would be either floating or on the bottom with no fins! He killed the betta within a couple of days, and the tiger barb murder took about 24 hours. I have the tank at work, and am losing about one fish a week. I plan to take the shark to the pet store and see if I can get credit toward a better suited fish for my community tank.
I also have a Red Finned Shark. All day he just sits in the corner under a large leaf. He is my least active fish. He never comes out except when the lights in the tank go out, or another fish chases him out. I have never actually observed this fish eating. However it is a very peaceful fish and his colors are stunning.