I am seeing a problem with this....
I have a similar sized fish, 11" Rose Queen, alone, in a 75 gal tank. Filtration is 2 X AC110's and one Fluval 304... therefore less bioload and more filtration than yours.
To keep nitrates under 40ppm, I have to do weekly 40-50% water changes (my tap water is ~0ppm NO3).
I have a very difficult time believing your nitrates to stay at 0. This is almost impossible.
I would bet that with those small water changes of 20-25% per week the nitrates are truely high (>80ppm), and HITH is probably the issue.....
Buy a new test kit, or get your LFS to test your water for you.
Burt