Had one in my 36g. He didnt hurt anybody. But like bettas they have personality. One might be docile one might be a bully but never kill other fish (well never might be stretching it but most of the time wont). They will mostly chase fish away from their territory and then retreat, therefore it is important to ensure the aquarium is large enough to allow the redtail some space to call its territory. I had mine for 7 years and caused no problem and my tank had a lots of fish for its size.
Redtail sharks prefer a PH between 6.8 and 7.5, a temperature between 22°C and 26°C (72°F and 79°F). They can reach a length of 6 inches but in rare cases they can reach 8 or even 9 inches depending on how they are fed. They can live up to eight years, although in some rare cases they have lived to be over 10 years old.
Oh and they jump, more so than bettas, that how i lost mine. Also they dont like other sharktype fish, thats one of the rare cases they kill other fish (including their own kind). They swim usually in the middle but they can also be bottom feeders and might harass others.
All in all they can be as pacifists as a clown loach or grumpy as a betta. If you love bettas you will love redtail sharks.