Hi all, I need some major help. I have a 55gal community aquarium with guppies, platys, tetras, and 2 Pictus cats. Yesterday I did my weekly water change and tested the water. Ammonia was 0, nitrite was 0, and nitrate was around 40 (have had problems with nitrates recently) the pH was low, around 6.4. It has tested that low for the last 3 weeks and I cannot figure out why or what's going on. I haven't added anything to change it because I'm scared of shocking the fish. Anyways, tonight my fish are acting strange and my tetras are dying. I've had 4 tetras die tonight and all of the other fish are acting strange- hanging out at the top, not active, breathing rapidly. What could've cause this and what can I do to save them??? I did nothing out of the ordinary yesterday. I did the same thing that I do every week for my weekly water change. Please help!! I would be devastated to lose all my fish 
