"Love at first sight" was the only way to describe how I felt after seeing the Redtail Balckshark for the first time. However, the warnings from books and web pages made me hold off purchasing one. Also, every reference made comment of having "only one per tank" due to their intolerable nature of each other, thus all the sharks I saw in the same tank at petstores had torn fins. Finally I saw one alone in a tank, its dorsal fin erect in full glory, so I bought it. A little over an inch at a half at the time, it's now grown to 8 cm from nose to base of tail in less than a year. The only real problem with any fish were with Tiger Barbs as the shark seemed to chase them with a passion, so I had to give the Barbs away. Although he still charges at my 2 yo-yo loaches from time to time and noses the Pleco out of his lair (a hollowed small piece of driftwood tree trunk), he has never killed anyone or for the most part bothered the Panda Corydoras catfish. All in all, I'd consider the shark a worthy addition to any aquarium as long as you discover their attitudes toward certain fish.
I've had Red Tail Sharks in every fish tank I ever owned. One grew almost 15 cm in my 600 liter tank. He was huge, he lived for about 5 years. Today (2/29/00) I just purchased a baby red tail about an 3 cm long. I put him in a 180 liter tank that has caves top to bottom and is well planted. I have a 25 cm pleco and a 13 cm jack dempsey in there. At first the jack was annoyed but after 6 hours the jack doesn't seem to mind him. I don't recommend doing this, but if he lives out the week there is a good chance he will be accepted in the tank. They are truly beautiful fish and great bottom feeders as well. Good for getting into those tight places. Every one who has an aquarium should try one of these fish.
Red-tailed sharks are my favourite fish. I own one in my 150 Liter community tank. He had an enormous growth spurt and grew from not even 3 cm to 9 cm in four months. He's not mean but bugs my black ghost knife fish a bit. He is actually very peaceful to other fish I have in my tank, even small tetras. I'm even thinking of buying a bigger tank so I can establish two territories and have two of these beutiful fish.
I have a 200 L community tank with one of these in. 'Blackey' spends most of his time in and around the flower pot. He guards his territory chasing away any fish who comes near. He is tolerant towards the others. He never hurts the others and he only swims after them until they are out of his territory. I have also noticed that as the fish grows it gets more and more cranky.
I've always loved the Red Tailed Black Shark, believing it to be one of the most beautiful fish out there. Unfortunately, as others have noted, some individuals are quite aggressive. However, in 80 liter aquariums, I have kept them in a tetra tank, swordtail and molly tank and a gourami tank. The little suckers have always seemed pretty hardy to me. The last one lived almost 8 years, and through some not so perfect conditions. Buy one, enjoy it, but just be warned, it's a crapshoot as to how agressive the one you buy will be.
I bought three red-tail sharks for my 110 L aquarium about 4-5 months ago. One jumped out of the tank and died and I don't know what happened to the other one. However, the one surviving is doing very well, has bright colors, and is not bothering the other fish. I suggest that you buy only one of these fish for each aquarium that you have because they tend to quarrel with each other.