A friend and fellow aquarist who's opinion I value told me he does'nt use carbon in his filters, so I did a huge water change on my slightly overstocked 150 gallon tank. During the water change I also cleaned 1 of 2 of the filters and didn't add carbon back into it. Since then I've noticed my Datnoids breathing rapidly, I have two seperate air pumps with airstones going in the tank so I'm sure its not lack of oxygen. Today when I fed them I also noticed my Orbignyi Ray didn't gobble down the food like normally. Last time this happened I lost a Ray because lack of knowledge of nitrates. So I tested my water and my ammonia levels was low, Nitrite was low, PH was 8.8, and my Nitrate was 40 PPM. I'm doing a 25 % water change now to bring down the nitrates but is there something I'm missing?? I really don't want to lose a fish..... How can I lower my PH, is that possibly the problem??