Whats wrong with my fish?

first one of my plecos died it had a couple of holes in its body i thought maybe the other fish nipped at it since i started giving them little fish to eat once a week..but then my other pleco died..my giant parrot fish is just sitting in one spot everyday not moving around…one of my silver dollars has some jelly looking stuff on one side..my synodantis catfish looks like someone took out chunks of top layer of his skin (just in few spots) (my other fish dont seem too excited either) i dont know if the fish i started feeding them were sick or had some bacteria but i just really need to get this taken care of before they all die out…i just cleaned out the filter and going to get some tablets to put in there..any ideas, input, or questions appreciated thanks