I have had this aquarium for about ten years but have never seen this problem before. I am losing fish slowly. the first a golden groumi looked like it had a raw spot on a side, the next a blue groumi looked fine one night and the next morning was gone and looked like it had bruises. I now have a black skirt tetra that looks like it has a growth on the mouth, and is acting sick. I don't expect it to last until morning. Anyone have any ideas

I do regular partial water changes, have some new artificial plants.

??