I just had one of my fish die from what I am thinking was fin rot. He was an oscar. His fins were missing in some spots and some of his scales were missing, too. He was also lying at the bottom of the tank and not eating. I guess he was just stressed out and maybe getting picked on by my other cichlids, because my water conditions are in very good shape. Well before he died I was treating the tank with meds. (Maracyns) for the infection. So do I still need to continue treatment in the tank after he died? Im not sure if it is contagious, my other fish look fine. What do you think?

how long did you have the oscar???

I doubt it was fin rot. probably just an un healthy fish from the store, that died slowly in your tank do to stress and poor previous living conditions. A quarantine tank is defiantly recommended.

That's what I told my husband but he insisted on having an oscar. My fish are just babies now but they will definitely grow. I hate when I get a new fish and then they die on me. Feels like I'm doing something wrong. Or maybe they just hate me. Lol.

Are you testing the water for ammonia, nitrites, and nitrates?

It's the #1 most important thing to do to keep the water healthy for fish.

Ammonia and nitrite should allways read ZERO.

Nitrates will rise quickly or slowely depending on how much bio-load (waste, uneaten food, etc) is in your tank. You should do enough water changes (not top offs) water changes, to reduce to nitrate levels to safe levels. Read the instructions. 10-20 is safe. Over 40 is possibly one reason your fish may have had fin rot or other problems.

Oscars are big eaters, and as fish grow they produce more waste and consume more food. You don't need to feed as much as the fish wants to eat. Feed sparingly, and unless you have bottom dwellers food should rarely have a chance to get to the bottom before it is eaten.

Non iodised salt is another good thing to add to a fresh water tank. It helps the fish ward off infections by helping them keep up their slime coat.

It depends somewhat what kinds of fish you have, but quite a bit will still result in a fresh water tank. Salt does not evaporate, so it should only be replentished when you remove water during water changes.

The amount of non-iodised (or aquarium salt) needed per gallon is on the directions of the salt sold for this purpose at stores. I'm forgetting the exact amount so I won't guess right now. Maybe someone else remembers.

I add about 1/2 cup to my 30 gallon planted, and more for my 100 gallon planted with gars, because gars need a slightly brackish environment to stay healthy. Too much will hurt some or most live plants more or less though.

Peterkarig is correct about the ammonia, nitrite, and nitrate. You should buy a test kit and see what your levels are at.
I do want to emphasize that the salt content in your tank is species specific. Different fish come from different home waters, so you want to match the salt concentration of the waters they would swim in in the wild. This may be more salt or less depending on where your fish come from. There's no way to prescribe a general salt amount without the amount not matching one of the world's water systems or another. *shrugs*

As to your tank, if the diseased fish is dead and the other fish don't show any signs of disease, then you should stop medicating the tank.