Hello, new here. I have had two mystery snails in a planted 20 gallon with fish. First platies fry, now 5 rainbow fish, 3 neon tetra and 1 platy. I got both snails at the same time. They grew well and all was fine. One day I noticed a rather large black, brownish spot on one of the snails. I had noticed that for two days prior she had been hugging the heater. In time, the spot turned brown and looked flaky like old bark on a tree. It looked like a spot in the middle of the brown was turning a lighter colour so I thought the snail might be fixing it but that was the only part that looked more normal. I moved the two snails into a 10 gallon with another mystery snail and a ramshorn. The one with the spot has slowed noticeably in growth and now the shell is starting to disintegrate. I had a cuttlebone in the 20 gallon and there was all different kinds of algea growing in there, brown, black, green, angel hair. I also fed them carrots and algea wafers. In the 10 gallon, I have been feeding them broccoli and zucchini since July 8. I have looked everywhere and cannot get any info on a black spot that does this. Help?