Help fish are getting sick

Question
I have had the hardest time keeping my fish alive for a long period of time. I have had my tank for about seven months now and have lost probably 10 fish maybe more. I have a self test kit that I check my levels and all are at zero. But I have one bala shark now with what started as a red spot on the back of his body close to his back fin which has now ulcerated and is a eatin area now. I have been treating it with melafix and it seems to be healing slowly. His behavior is normal. But yesterday I noticed one of my catfish acting sluggish and tonight I was watching him and he had a red area in his belly like he was bleeding internally. He was stuck in a corner obviously dying so I pulled him out cause I couldn't stand to see him suffer. But today I just added two silver dollars, one clown loch, and one serpea tetra. I already have two rainbow fish, two bala sharks and one serpea tetra. I did not realize my catfish was dying or I would not have added any new fish. Anyway these red spots are not the first time I have seen them on my fish. This also happened to a groumi I had a while ago. I had him for about four months when he all of a sudden developed this red ulcer on the back side of him and eventually died. I just don't' get it. I feed them freeze dried worms which everyone is telling me it is healthy but is it really. Also if this helps any I had a kissing groumi and I found him dead about three months ago with a bruise on his side of his belly. The fish store said that happens after they die not before. I am trying to give you as much information as I can cause I don't want anymore of my precious fish to die. I do have live plants in the tank and I do add the iron, flourish, excel and one other plant nutrient. I clean the tank about once a month with a 10-20 percent water change. It is a 37 gallon tank. I can't think of anything else you may need to know. But I sure hope you can help me.
Thank you so much in advance for any help.

Answer
Hello. It sounds like your whole tank is becoming infected with the unknown disease. I suggest removing the infected fish into a separate tank. I am not sure what this disease is but it would be safe to buy a general all-in-one parasite medication. When looking for this medication look for red spots or possible areas your where your fish have the disease (i.e. if it says it treats the spine or back of the fish and thats where your fish is looking ill). If the Melafix is still working continue treat as directed. Take out the carbon in the filter also because if you don't the medication wont do anything. In the tank with the fish who don't have the disease also treat it to prevent them getting the illness. And a cure for almost everything is salt. It helps heal wounds and kills off unwanted bacteria and parasites. I don't know how much salt you should add considering I don't know the disease. In the normal tank put about 5 teaspoons of dissolved salt in the tank. Do the same to the other tank to see if it helps any. Well those are all the cures I can think of. Freeze dried bloodworms are an excellent food for fish along with freeze dried krill/brine shrimp, or plankton. I hope I helped! Good luck! I hope every thing turns out ok!
Corinna