One Axie dead, other taking turn for worse..

I started a thread earlier titled shrinking, shriveling gills, etc. and mentioned that one seemed to be getting better...I was mistaken.
One was just going down faster than the other.
I discovered I had a Nitrite problem and fixed that immediately. I have been doing 50% water changes daily. The other two in the tank seem fine other than some gill flicking (don't know if that is bad or not..)
Here are the physical symptoms of both.
Dead one: (for informational purposes, please don't be offended at the impersonal nature of the descriptions, I loved him very much!)
Shrinking gills, refusal to eat, lucistic turned even whiter, stayed on top of water, gills lost all color and fluffiness, and shrunk to about 1/4 inch. as it progressed. Even with clean, perfect water, progression went as follows:
He refused to dive at all & hung on to floating plants. Severe lethargy, I could handle him and move him around by hand, and he hardly reacted. Soon afterward he turned very yellow and was dead less than 12 hours later.
Live one:
1st- Shrinking gills, loss of appetite, not very active, gulps air.
2nd- (fixed water problem) color comes back to gills, feathery texture begins to return, more active but will only eat an occasional cricket or partial worm.
Currently- Refuses to eat again, gills seem to still be recovering, but his tail has become pink. He is still gulping air, but less frequently.

I'm worried that he will starve to death. I don't like the pink in his tail. I don't have a clue how to fix it. The only medications that I currently have is Melafix and Methyl Blue. To be honest I don't know how to use either.
I don't want him to turn out like his brother. My other two are fine and until 2 days ago, they were in the same water.
Water temp is 69, Ammonia 0, Nitrite is now at 0 and has been for about 7-10 days, Nitrates come out of the tap here at about 20 and it is a struggle to keep it below about 35.

I don't know what is wrong with him, so it is impossible for me to attempt a treatment. I need a diagnosis PLEASE!
Is there anything I can do? He is so skinny!

Perhaps putting him in the fridge? Ive seen on many of these posts that people suggest putting them in colder water when they are sick, but it has to be above I believe 5 C. Read through some of the other threads for suggestions on keeping them in the fridge.

The pink color on the tail is worrisome, as this suggests bacterial infection. I don't think Melafix does much good. But it's impossible to say what might work. You can certainly try refrigeration, or pet shop antibiotics that they make for fish, but I can't say if any would do any good. It's a shot in the dark.