Yes it can be contagious. When the Cls pass the internal parasites other fish can eat them and become infected. I get a while back several CLs from petsmart when they were on sale. They were very skinny and I have successfully treated them in a separate tank with Tetra Parasite Guard. It worked great. The active ingredients are Praziquantel, Dflubenzuron, Metronidazole and Acriflavin. Now the CLs are fat and healthy. This medication worked great for me for these and 2 others that I had gotten. I would suggest if you have these in your tank to treat the whole tank.