I recently got a small red-tail shark from my local pet shop. At first he did all right in his 40 liter tank with assorted tetras. After a couple of weeks, though, he started chasing the other fish endlessly. After receiving an emergency transfer of 4 danios from a tank where they had been eaten up, I decided to move the little red-tail from the now over-crowded 40 liter to the Asian 110 liter. When I moved him, the gouramis in the tank acted aggressively at first, then they let him be. He now swims happily all day, happily cleaning the algae off the sides of the tank.
I have had a red tail shark for about 4 months. When I first bought him, the pet store where I got it advised me that having more than one in a tank at the same time is not a good idea. When purchased he was about 6 cm, he is now about 12 cm. I have recently put him into a 170 L tank. He is not aggressive to the other fish except to protect his lair. My other fish include various tetras, neon tetras, bala shark, angelfish and various catfish.
Red tailed sharks seem to be a bone of contention as I have also read people saying that in a shoal they are fine. I have 5 in my 120 litre tank of differing ages and sizes, from one of 15 cm to 3 babies. Yes, they harass and chase each other, but they seem to establish a chain of rank after a while, with the most dominant growing fastest but the others not apparently too distressed - certainly no torn fins. Of course this can change, but my only problem so far is saving for a bigger tank as the biggest one is already far too big for a 120 litre.
I have a red tail shark that is 15 years old, and measures 14 cm long. This fish owns the tank. The only other fish in the tank is a tiger barb. His home is a 80 liter high tank. My red tail shark does not liked to be bothered, so the tiger fish has learned to stay in his corner of the tank or it will get chased back to it's corner.
I purchased a couple of red tail black sharks for my 470 L community tank. They share the space with 45 other fish inlcuding tetras, danios, angelfish, gourami's, tiger barbs and a bala shark. I've had the tank seven months and I recently gave one of them away as it was constantly harrassed by the other. The only fish any of them ever chased was the tiger barbs, which were always chasing each other around anyways. It has been an otherwise peaceful fish that pretty much minds its own business. And is a wonderful addition to the tank.
I have a male red tailed black shark (7 cm) that happily shares his hollow log with a 22 cm Dragon Fish. He will swim upside down under the log as the dragon fish lays on the bottom. Also shares the 130 L with a pink kissing gourami (5 cm) and a plecostumus (11 cm).