Posting this on behalf of a friend. He has 2 Angel fish that have lost a lot of colour and possibly their slime coat. (I’m not sure from the pictures) There are no other fish in the tank. He has a 55 gallon tank with 2 hob filters. LFS told him his water was good. However I told him to buy his own api test kit. Parameters are ph 8 ammonia 0 nitrite 0.25 and nitrates look to be between 40-80. I know that nitrite should be zero and the nitrates a lot lower too. He’s currently performing a 50% water change after doing a 25% yesterday (which indicates they were higher than what was just tested). They’re eating and swimming fine. Could the nitrite and nitrates be causing the colour loss and shedding of slime coat. He’s just swapped out his gravel for sand in the last 2 weeks too, could it be a crashed cycle? Pictures below for reference. The last one is how they look now.