how long have you had these fish in the tank?
could be useful to check water parameters, ammonia, nitrate, nitrite etc
if these are too high the fish wont be happy
they could also be sizing each other up ready for a fight, the red tailed black for sure is very territorial and may attack another shark looking fish
There are some great articles pinned to the top of this section of the forum. I'd start reading there. I'd also get some test kits if you don't have them already. If you don't have any and can't get them right away most local fish stores will test your water for you. Just bring a bit of your water in a ziplock bag to the shop. If your tank is not big enough to accomodate the fish you purchased I'd bring them back and start again with something smaller.