help with 55 gal tank

[kildragon6]

hi, ok so ive noticed that one of my pacu's has no color from the gills up, he wasn't like that when i bought him and has been like this ever since. my tank has two pacus, 1 about 5 in, and 1 about 1.5, 1 tinfoil barb about 5in, 2 pictis catfish about 3 in, 1 common pleco about 6 in and an gourmi about 4.5 in, the tank is well overfiltered, it has a emporor 400, and two of the walmart 10-20 gal filters, the water is clear and all readings are normal , except the last in the cycle i think nitrates, i had a very bad problem with the tank being extremly dirty on the bottom as the tank had been neglected for a lonnggg time befor i took control of it, i gravel vac the whole and have since done many wc over the past month but my pacu still has no color? they all eat very well and are very active. and they will all be in a 180 very soon, i have the tank just need the filters and such but should i worry about his discoloration