black ghost knife sick

Please help :( Vincent is a 7yo black ghost knife in a 75 gal two over the back filters (I know canister is more optimal) all alone (I know this is the absolute minimum size but for now let’s just make him better) 28/29cel Nutrafin waste control as recommended Gh around 180 Kh around 60 Ph...

Hi all   Background - I've had a BGK for 4/5 years since it was 2 inches long, it now lives in a 56 gallon tank with fluval 305 filter, kept at low temp around 21c current water readings are ammonia 0 Nitrite 0 Nitrate 25 (did a 25% water change last night to try and get this down, doing another...