That was easily treatable. It was lymphocystis. It is cured by feeding them high quality foods dosed in a rotation of Vitachem and Selcon. Keeping your water parameters in check helps keep it away. It is a viral infection. If the fish is healthy then they will not get it. It is similar (not really, but comparable) to a cold for fish. You fight it the same way. Keep the immune system high, reduce stress, etc. They probably got them in in poor health. It is a good idea to always feed your fish a high quality diet dosed with a rotation of vitamins. A healthy immune system for fish goes a long way. Believe me, it does. Keep your water parameters pristine as well. Poor water quality will make them fall ill.