what to do?

Question
I used to have two snow white socolofi. Their tank kept growing this yellowish gooey substance with white flakes. The only possible causes of death i can find is the gooey stuff or they got to cold. I was wanting to know if you know what the gooey stuff is, and how i would clean the tank for new fish. Also, the only filtration system i have is an under gravel filter, and i cleaned it out every two weeks.

Answer
Zy,

Most likely it was caused by the under gravel filter. These filters are more trouble than they are worth. Every time you clean them you are dropping all the bad stuff into the tank. I would replace it with a filter that can hang in the back of the tank or suction cup to the inside. As for the cleaning, the best thing to do is take it all out and clean it very good with very hot water. Do not use any soap at all because that could kill the new fish you put in the tank. I am sure that if you replace the filter, you will not run into that problem again.