Okay I've come to the end of my rope. I have been up all night trying to save my aquarium and I am now reaching the limits of what I can do with my current knowledge.

Here is the problem
I began my tank maintenance two days ago (I spread the work out to avoid over stressing the fish). I started by cleaning the algea from the front of my tank glass and then last night I rinsed out the filter before I headed out for the evening. I got home and the water had turned yellow and two of my fish were gulping air at the surface of the water. I was concearned to say the least so I immediately vacummed the bottom of the tank and treated the water for amonia. Once that was done I also skimmed the surface of the water for slime.
This seemed only to agrivate the situation as the water became cloudy and the remaining fish began to gulp air as well. To make matters worse my Flying Fox died and the clown loach was showing signs of passing on as well. I did an immediate 10% water change (bringing the total up to 20% for the night). For a short time this seemed to work and the fish began to recover, but the tank continued to cloud over. Things began to deteriorate again and I was forced to do another 10% water change and then skim the water further for slim which had began to build up rapidly. I was forced to remove the Loach and put it into a quaranteen tank (water was 50% new treated water and 50% old water). The loach not only recovered but showed unpresidented signs of life. I then removed all items from the tank and cleaned them thoroughly (they have not been added to the tank yet) and I vacummed the tank once more, did another 10% water change on top of that and skimmed for Slim as well as a second treatment for slime. If I do any more water changes I am going to risk Ph shock.

The tank has been running for at least 3 years and I have had many fish in my past and I am completely comfounded by what is happening. I have read that this may be a bacterial infection but I am loath to us anti-biotics at this point. This is beyond my knowledge.

Tank size: 20 gallons
Fish: 3 Bala sharks (about 3-4 inches long, I know that I am going to have to move these to a larger tank, I just don't have one yet) and one small clown loach.
Plant life: Was obliterated by Flying Fox... seems odd but I watched him eat the leaves on several occasions.
Water filter is on

Fish are begining to try to jump out of the tank...
I am executing a complete water change. Removing fish to fresh water has rejuvinated them. I will replace water and then hopefully they will be okay.
Well when I finished responding to the first answer I turned around and in the few moments it took me to type my note my fish looked like they were all about to die. I decided that drastic action was needed. At the time I only had the first answer and a complete water change seemed like the most likely way of saving my fish. Indeed getting them into an old (and very much clean) Ice cream bucket they began to recover within moments. Three of the four remaining fish survive now in a Ten gallon tank I am using as an ad-hoc enviroment (there is nothing else in it) after getting the water out of the 20 gallon I observed that the algea seemed to be secreeting the slime. Its black with a slight green tint. I have never come across this stuff and I am able to confirm that the algea is the culprit because when I scalded the drift wood slime seeped off it in rivulets. I am contemplating the future of the drift wook.
Unfortunately in my haste I have failed to save a water sample for testing :(