I got new fish at Walmart and one, after about a week or two died. One of them, lost there color after I found them dead. The other one, was found in a white sac. Please help!

Additional information: ten gallon tank with top fin filter. One molly, two red tailed tetras, three black shirt tetras, and a small pleco. I tested my water and every thing is fine.

Thanks, I hope you can help!