Red Tail Shark coloration

Question
In my 25 gallon, I have a red tail shark along with 2 swordtails, a beta, a farlowella, 2 rummynose tetras and a corey. My red tail shark is very pale. Instead of the deep black he is supposed to be, he is more of a gray and his red tail is very pale pink. I just had my water tested and it has really high nitrate and GH. I am in the process of fixing this with water changes, etc. Is this the reason my red tail is losing his color, or is there another problem I should look into?

Answer
First I need a little more information.

Does your shark have only a red tail or are it's other fins red as well? Because a red tail shark and a rainbow shark look very similar but the rainbow sharks have every fin (dorsal, anal, tail, caudal, and pectoral) with red coloration and the body tends to have more grey.