fish/white cotton ball growth

Question
I have a 40gal tank and have had it set up for a long time (more than a year). I have had fish for about 10 years but I have never encountered this problem.  About two weeks ago I cleaned the tank meaning I vacuumed about 3/4 water. Since I've got this cotton ball like growth on the rocks, and my fish are congregating at the top of the tank like they need air.  What am I to do???

Answer
Hi Tab,

Are the cottony growths on the fish? If the fish look normal, but just gasping for air (rapid gill movement), there's nothing to worry about.
Perform 20% water changes every other day, and vacuum the gravel along with it, until the problem clears. The cottony-like growths are caused by poor water quality/lack of aquarium maintenance. Never change more than 30% of the water at a time. Changing more will harm the biological balance in the aquarium, or in other words, it will kill the beneficial bacteria that removes waste.

As a general maintenance schedule, water changes should be done approximately every week, with a gravel vacuum and a 20% water change.

Add an air pump and an airstone to the tank - the reason the fish are at the surface is because of a lack of oxygen, not related to the cotton.

If there's cottony growths on the fish themselves, its a Columnaris bacterial infection. Treat with an anti-bacterial remedy such as Maracyn or Maracyn 2, by Mardel Labs. If the cotton is just on the gravel/rocks, it's not an infection.

Good Luck, and Happy Fishkeeping!