I have a 20 gallon filtered and heated tank. It's been running for nearly a year. All water parameters are normal 0-0-20, PH is 6.4 I keep driftwood and plenty of plants, Ocelot and Amazon Sword, Java fern, Java moss, Banana Lilly, etc.. And my 14 Neon Tetras and 1 Clown Pleco that were moved from my 10 gallon to this tank about 6 month's ago. Since that time this tank has temporarily housed other fish such as my Zebra Loaches until I was able to move them to a bigger tank. And 4 Cobra Endler Guppies that still remain in this tank. A few weeks ago my neon tetras started looking and acting strangely. They went from bright vivid coloring to pale grey and what looked like white fuzz on their bodies and mouth. They stopped swimming and would hover in one spot. I noticed some rapid breathing and slight twitching also. Then they started dying at a rate of 1 or 2 per day until there were none. During that time I stepped up water changes but that didn't help. I thought my Clown pleco and Endler Guppies would be next to go but they are fine and appear unaffected. The tank sat like that for a week and I thought it would be a good idea to move my school of 20 Ember tetras, a female betta and 2 more Cobra endler Guppies that I've had all for over 6 month's to the now empty looking 20 gallon. 3 days later and now the ember tetras are dying off at a rapid pace. But none of the other inhabitants are being affected. The symptoms are rapid breathing and difficulty swimming with head either pointing upward or bodies tilted or upside down. I've lost over half of them in last 3 days. What's going on?