I have a 20 gallon tank. We went on vacation for a week, the in-laws extremely overfed our fish. The fish weren't fat or anything the water was just a horrible dark brown. I did water changes until it was clear again. Took care of the tank for a week, and then had to go out of town for 2-weeks. My wife fed the fish during this time. For some reason half of the fish died, and one of our snails died. One of the fish that died after I came back was ballooned up to about 3 times his normal size. None of the other fish that died balloned up like this. None of my x-ray tetras have been showing any signs of being ill, its been the other fish so far. Except now one is having trouble floating, he's always sinking, still swims around and eats though. I've been feeding them a varierty of sinking food, flake food and every now and then brine shrimp.
Ammonia, Nitrite, and Nitrate levels are all fine, and this is an established tank. Could there be something wrong with my tank?
Answers:
The problem is due to the bad water condition that happened when you were away. The bad water condition probably weaken the fish's health and bacteria took the opportunity to infect the fishes.
Since NH3,NO2 and NO3 is stable and in a good range now you should try to maintain it that way for the time being. Hopefully no more fish turn up sick but I am quite sure no more would if the water parameter is kept in the good range.
I would recommend that you not add more fish for the next two week and keep monitoring your system. After that roller coaster ride with the water condition, the fish would be stress and we don't want more fishes to mess up the water for now. IMHO stability is the key for the next two weeks.
Do not add medication if non of the fish look or behave sick. Even if they're sick, you should take them out and treat them in a hospital tank. That way if you use medication that kills good bacteria, you wouldn't mess up the main tank. And also that will prevent the disease from the sick fish from spreading to the others.
A little salt in the water is fine for most fishes.
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it sounds like your existing fish have been overly stressted. this causes thier immune systems to weakin. this allows existing problems come to the surface and cause harm. i probably wouldnt add anybody else for a couple weeks just to see if anybody else is gona get sick.keep up on your water changes
