white cotton like substance on gravel

Question
I have a 35 qallon tank with about 15 tetras. I've had the tank for 10 years. About a month ago a white cotton looking substance starting growing along the gravel. I pulled a 50 % water change and vacuumed the gravel. The white stuff disappeared and came back in about a week... can you help?

Answer
I have had this problem as well. I switched my normal gravel to aquarium sand once and within a week there were clumps of white spots that looked like cotton. Well, I did as you did and did a water change and cleaned the sand again. However, to my dismay, I also imagine to yours, the nasty white cotton came back. Well, I am sorry to say the only way I was able to get rid of it was to completely replace the sand I had with brand new gravel. I think this comes from gravel or sand not being stored in the right location. For example, the pet store may store thier gravel and sand right under the fish tanks, this is a major bad thing to do. The humidity from the tanks gets into the gravel and sand and breeds mold. Once the gravel or sand is in a tank it just grows and grows. Since it is not a fish disease or parasite there is no way to treat it and the only way to get rid of it that I have found is to completely change the substrate, gravel or sand, to a brand new substrate. However, since tank substrates ae pricy sometimes, I suggest completely removing the gravel and washing it out with freshwater from the tap outside or the tank. This may help to eliminate it, but keep in mind I have never been successful with this so I do not know if it will work. There are no signs of disease or parasites on the fish right? Assuming no, you just need to aggressively deal with the substrate. In summary, you can either continue to do periodic water changes and gravel cleaning, which will be very stressful to the fish...or you could completely replace the gravel with brand new gravel, which is the pricy option...or you could completely remove the gravel and wash it out, outside of the tank, which is a more labor intense method. Other than these three ideas, I honestly do not know how to fix it since it is not something I have ever seen a treatment for. If these ideas will not work for you I suggest asking another expert, as they may have other ideas. I hope I have helped you some. Hoepfully you will soon be rid of the nasty cotton on the gravel. Feel free to reply if you have any more questions or comments.