I have had my Red Tailed for nearly a year now and it's doing great! Mine isn't aggressive at all but like all the others can normally only be seen at night (or when the lights not on in the tank). I've been feeding him my own special mix of Tetrafin, and some other treats for his colors once in a while. I think the food is helping a lot, because his scales are just about the blackest you can get and his tail could be considered crimson!
I have found that my Red Tailed Black Shark doesn't have a red tail to start with. I got a very young Red Tail in my 150 liter aquarium a while ago. The shark originally had a dark tail, which developed into a peach colored tail. Eventually the tail became bright orange. When the shark was young it was docile, but after a year it became aggressive towards other fish. I have had a Red Tail that was so aggressive; chasing around Tiger Barbs, Tetras, etc, that I had to return it to the pet store before it stressed the other fish to death. At one time I had three Red Tails in a tank together. They were constantly fighting, and their fins were torn to shreds. I had to remove two of them so they wouldn't kill each other.
I just bought a Red Tail Black Shark (very large 13 cm) from a local pet store. He was placed in his new home with 3 Oscars, the biggest being 15 cm in length, there is also one 10 cm Bala Shark. My aquarium is 470 liters. This little brat immediately chased and harassed all the fish in the tank. The Oscars, to my surprise, were shivering in terror. Well, to make a long story short I love this fish and he now lives in his own 110 liter tank and is doing great. It is a beautiful fish and I recommend getting one, they are highly active! The pet store owner told me he was a trade in - guess I should have asked why he was a trade in - LOL.
Note to those out there that get Red Tailed Sharks...I bought one, put him in the tank and he chased my Green Tiger Barbs until one day I found my poor Red Tailed Shark ripped up. Now who do you think did that?!! It was either the Green Tiger Barbs or the Tiger Barbs who would intentionally anger him and hide in the rock or tires I have, then race out, circle him, ram him, nip him, and then run again.
Bought 2 sharks originally for my first aquarium (50 L) - I didn't realise the size they grow or their belligerent nature. One bullied the other to death and is now 13 cm long, and it chases the other fish around the tank at feeding time - he's getting moved to a 180 L tank - not looking forward to trying to net him.
I have owned 3 Red Tails over the years. My first one was in a 75 L tank, along with some Rosy Barbs, Silver Dollars, Tiger Barbs and a Chinese Algae Eater. I got the Algae Eater and the Red Tail at the same time. Both were 5 cm long when I got them. I never had too much trouble with them bothering the other fish, until they got bigger. They hung out together like they were buddies, maybe because they went into the tank at the same time. When it was fish chasing time, they did it together. They never killed any of the other fish, but some of them were pretty ragged looking. Needless to say, they both got huge. I ended up having to take them back to the Pet Store, where they put them in a 200 L tank.