Fish wasting away

Question
Hi, I have a ten gallon tank, and in it I kept 4 fish.  2 fantails that had a baby, and a black fish that ate the algae and lived under the decorations.  About a 2 weeks ago the black fish died.  I noticed his dead body about a week ago.  My water was very dirty the week that he went unnoticed so I changed the filter but that didn't work.  So anyway the night I noticed the black fish was dead I looked at the daddy fantail, and he had red gills and there were 2 red streaks going all the way to his caudal fins.  He was struggling for air.  He had drammatically lost weight, and the curve of his back had become flat. I immediately transported them into a fish bowl, and put oxygen into it too, to help the daddy fish. I figured he was just having stress and trouble breathing because of the dirty tank.  I then completely redid the tank.  Bought new gravel, new decorations, scrubbed down the tank, and changed the filter.  I then let the tank run for thirty minutes and then added the fish back into it.  The daddy fish just went to the back corner, laid down, and kept opening it's mouth.  I thought it would just need a clean tank.  I just kept them under a close eye for the next two days, and then due to my grandmother being in the hospital, I left and left my mom in charge of taking care of them.  I came back home four days later, and now I see that his fins are shredding apart with a whiteness on the edges, and his scales are falling off.  I've looked online for hours, and all the symptoms they say are some but not all, or sort of, but not exactly. I don't know what to do, and I just want to help him, so any advice that you have would be wonderful.  Thank you very much.

Answer
Hello Marina-

Terribly sorry for the long wait, my grandfather just got out of the hospital and things have been busy for me.

What I would recommend is this:
1. Isolate the male fish in a smaller tank. Use fresh clean water, not the water from the tank. Also, try to catch him with your hands, but be careful not to squeeze him.

2. Add a small dosage of aquarium salt (or sea salt, but NOT table salt) to the hospital tank.

3. Add a dosage of a fish medication that contains formalin. You can buy a concentrated dose at a pet store, or WalMart has a brand called "QuickCure."

4. (optional) Treat with a strong dose of Melafix to promote scale and fin regrowth.

5. Treat your little guy for three to five days. If you do not see any improvement, switch to a medicine called Maracyn II. This is much stronger, and I usually recommend it as a last resort.

Again, terribly sorry for your wait. I hope we can still help him!
-Amber Worman