gasping not eating fish

Question
QUESTION: Hi, I have a 75 gallon tank.  We have a ehiem filter for it.  Yesterday we hooked up a second filter (Fluval). We didn't put the carbon in, incase that matters?  The next morning all the fish are breathing heavily and some even gasping up top.  Not eating, when we stand in front of the tank, usually they react, but not today.  Tested the water for ammonia, nitrate and Ph and they are all fine?  Which even makes this more confusing? Any ideas what could be wrong?
ANSWER: Hello,

You mentioned that you tested your water for ammonia, nitrate and pH. Did you also check your water for nitrite? This is the second stage of the nitrogen cycle and is just as deadly in high amounts. Usually when I have a water-quality issue it is caused by nitrites, so make sure you check them.

If that is not the case, then you'll have to try a process of elimination to see what is wrong. Is enough oxygen dissolving into your water? Do the fish appear to have a disease? Does the water appear clear? Is the temperature remaining consistant? Did you add any chemicals recently? If so, did you add too much? Have you been cleaning the outside of your tank or around your tank with any foreign chemicals that may have gotten in your water? Did you treat any water you added with dechlorinator, if necessary?

Try and go over everything that may have happened to your tank since you added the new filter. And check for nitrites. That may be able to help you discover what may have caused this to happen to your fish so quickly.

I hope this helps.

Regards,
Stephanie

---------- FOLLOW-UP ----------

QUESTION: hi there, eveything in the water is perfect.  I took apart the  second filter and put it all together again, incase i did something wrong.  Right when I turned it on, within an hour i get the same reaction from them.  I was looking on line, one website said to add some old media from first filter to the second one. Would that do anything?  The only time they feel better is, if I do a water change, there good for a couple of hours then back to the same symptoms.  Usually they always beg for food when i'm in front of the tank, but not now.  It would help if something was wrong with the water then i could fix it.  Since the water is fine, it makes it more difficult.  

Answer
Hello,

Adding filter media from your old filter to your new filter may help increase levels of beneficial bacteria in your tank. Since you have assured me there is nothing wrong with your water, then this is likely not necessary.

If it is not a water quality problem then it is probably a disease. That is the only other thing that could cause fish to behave in such a manner. There are three main types of disease:

Parasites will appear as white dots or worms all over your fish. They can be treated with a product called Coppersafe, among others.

Fungus would appear as white, cauliflower-like tufts on your fish. I recommend Pimafix for this.

Bacterial infections come in many forms and symptoms. Based on the gasping that you described, I think an internal, bacterial infection is most likely what is ailing your fish. I recommend a product called Maracyn-2 for this.

I hope this helps.

Regards,
Stephanie