I had that happen to my guppy tank and don't know what happened either. I treated for ick and other parasites. I increased my oxygen and turned off lights for 5 days. I removed the carbon from the tank and after a long uphill battle My tank is finally comeing back around. It took months for me to get control again. I also have a 150 gal tank that they are in. I think if it were smaller it wouldn't have taken so long! I am sorry for your loss and hope it doesn't take as long for you as it did for me.
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Tank: 75gal, 12gal LifeReef Sump, LifeReef Skimmer, 2 Koralia PH #2 and #3, 300w Heater, dual 175 mh with 2x 65 watt actinic, 78lbs LR Friends: Maroon Clown, False Perc Clown, Pajama Cardinal, Diamond Goby, Kole Tang, Starry Blenny, Watchman, Pistol Shrimp, Urchin, Sally Lightfoot, 2 Cleaner Shrimp Corals: GSP, Sun, Torch, Pipe Coral, Duncan, Galexia, Pulsing Xenia, Red, Blue, Superman, Ricordia Mushrooms, Finger Leather, Polyps, Toadstool, Flower Pot | 
guppies 
