We recently lost an albino cory catfish, a white molly, and a danio in my 55 gallon freshwater tank (their bodies had some lesions and red lines on them) I thought my ghost shrimp were attacking them and eating them since I found the shrimp eating their dead bodies. I had my water tested at the local pet store and they said my water levels/readings were all really good. I noticed one of my black skirt tetras had a white dot on its nose so I assumed I should treat for ick, which I did. Then I noticed the same fish was losing its back tail, and a neon tetra was missing its top lip. I started the next day with API Pimafix and Melafix. Its tail was still rapidly disappearing next day (fin rot), so I added EM Erythromycin to the treatment. The neon tetra died and so did my black skirt tetra (the fish basically dissolved into nothing). Now I'm going to complete the 7 day treatment with Pimafix, Melafix, and 4 day treatment of EM Erythromycin to ensure all the other fish are ok. My question is, do these medicines rid the entire tank (gravel, decor, live plants, water) of fungus, bacteria, whatever disease is in there? Or do those only treat living fish bodies? Do I need to use a parasite treatment or other medicine? How soon should I wait to introduce new fish? I had all those other fish for 6 weeks +, but added some small neon tetras a week prior to the few that died (assuming they were carrying a disease even though none of the new guys died). The only other thing that's different is I started feeding my fish freeze dried blood worms, could those possibly carry disease? Pic attached, sick tetra on left, "healthy" on right.