I have a 75 gallon fowlr
tank. Have one tomato clown, one javanese damsel, one blue damsel, one cardinal fish and one yellow tang. Also have one long-spined sea urchin. Tank has been running for about 8 months. Yellow tang was last fish added about 3 months ago. Today the tang is acting lethargic and has a little poop hanging out that he is not expelling. All other animals are acting normal. Tang was acting normal yesterday. I did notice the water temp was about 2 degrees lower than normal so put a heater in the sump. Ammonia read 0.25 (which seems strange), nitrite = 0, nitrate = 15. In the process of doing a water exchange to get parameters better. Tang is still slowly swimming but did not feed today. Any suggestions?