Help with flowerhorn please

[Slothnes]

Yesterday I noticed he wasn't eating well and today when he swam out I noticed a black spot area around his butt. Any ideas on how to help him?

 

[Rocksor]

test tank water with liquid tests for ammonia, nitrite, nitrate, and ph.

 

[Slothnes]

Nitrates are surprisingly high. I wasn't expecting that reading at all

 

[Rocksor]

First thing to do is bring the nitrates down to less than 20ppm. If you have been doing weekly water changes, you should be able to do 2 75% (150%) water changes back to back to see a reduction in nitrate. Once that is taken care of, watch the fish for a few days to see if you have signs of improvement.