Hi, These are probably what is referred to as planaria. Do they often stick to the glass at the water line, if yes then they are planaria. I believe that they are there as a result of overfeeding and although they may be harmless in themselves the effect on the fish due to water quality may not be so good in the long term. Try not feeding your fish for a few days and the should decline in population. I used to observe these creatures in my Discus tank when feeding heavily to encourage growth.. Hope this helps, Ken. > "Pierluigi e Simone Vicini" wrote: > <<<<<Subject: white little worms > I found, during my water change, some very little worms floating around > in > my tank. They really look like Tubifex but they are totally white and a > little smaller, I think that this may become a big problem for my fishes > because there were hundreds of them. > Is there a way to kill them and how did they get into my aquarium>>>>> >