Deadly tank water

[Optimist]

Hi guys. I haven't posted on MFK for a while, but I really need some help fixing my 29 gallon tank. A couple months ago, water conditions got really bad and killed a couple of my fish. No matter what I try to do, the problem won't seem to go away. I've tried 100% water changes with heavy cleaning and recycling the tank, but I added a red devil today and within an hour or two, its fins were rotting and its eyes were cloudy (see pictures). I have since moved the red devil to another tank in hopes that he will recover. Nothing appears to be noticeably wrong with the tank. As you can see, the water is crystal clear. Can somebody please tell me what is wrong and how I might cure it? Is it just standard fin rot, or some kind of fungus or bacterial infection? I really want to use this tank as a quarantine/hospital tank, but it is in no condition right now.
Here is the red devil with his fins beginning to rot:

Here is the extent of the damage after I moved him:

Here is the problem tank:

 

[Mattyou]

Check the following,
Water params / water treatment (liquid to remove chemicals) / low bacteria = high nitrite / ammonia.....about all I can suggest.

 

[Onion01]

sounds like high ammonia to me

 

[deepblue101]

damm chage the water!

 

[doc-55]

Okie-doke, The first thing I would do is check the source...Or the water coming out of the tap, check just as you would your tank water. second I would think there is no need for single large water changes, try 30% changes 3 times weekly......If indeed you are doing 100% changes and scrubbing everything you might be getting yourself into mini-cycles or the like...Relax and start from the begining..........Check your filtration and oxygenation.....do a set of tests IE Ammonia, Nitrite, Nitrate and Ph, KH, GH if you can post the results and lets see what we can do. I do not know were you live, but where I am from my aquafer water is hard and very alkaline, so my ammonia will build up quicker at higer temps........put the info out there and you will get the advice ya need.........hang in there bubba....Dave