Giant Danio disease

Question
I have a 15 gallon tank with a pleco, 2 Giant Danio's and 4 Black Tetras. I hadn't lost any fish in two years until recently. I lost my other two Giant Danio's about a month ago...their mouths seemed to gradually disappear, and one of them had a cloudy eye. Things were fine for about a month and now my remaining Giant Danio's are developing the same problems, one has his mouth disappearing and the other has a cloudy eye. The rest of their bodies are fine, no white spots. I've read about something called mouth fungus, but I can't see any white threads in their mouths...they just seem to gradually disintegrate. They are still eating regularly and are behaving normally. All of the other fish in the tank are fine, whatever disease it is only seems to affect the Giant Danio's. I do water changes (25%) once a week. Any ideas, or is it too late. I'm concerned about my entire population being wiped out.  

Answer
Hi Kevin,
The first thing I  think you should do is to test your water for high ammonia, nitrites and nitrates. If they are high you need to do more water changes. Sometimes, when there is a pleco in a small tank they will pollute the tank water extremely fast. If there is high ammonia or nitrites it can cause fin rot and possibly the problem that your fish have with their mouth. You could also try using Melafix and Pimafix to help heal and prevent more damage. Hopefully, they will survive.

Make sure you keep up with the water changes and it might be a good idea to remove more water and be sure to siphon the gravel since it will often contain decaying food and fish waste that will also cause water conditions to deteriorate.

Best of luck. If you have more questions feel free to write back.
Hope this helps,
Karen