I'm three weeks into cycling my tank with Guppies. I've noticed that two of them haven't seemed as healthy as the others. These two I bought from Wal-Mart and the others elsewhere. Today I noticed a patch of cotton-like fungus on it's back. It swims kind of labored and prefers to be floating at the top of the tank or laying on rocks at the bottom. They both have little tolerance for the aquarium light when it's on more than 10 hours. They hide and look dead until I turn the light off - then they perk up. They do not look very healthy. Any suggestions? Should I return them to Wal-Mart? My biggest concern is them contaminating the entire tank, thus setting back my cycling and making my remaining Guppies sick (one is pregnant
) What should I do? Is this disease contagious? Are my other fish going to get it? HELP PLEASE!

