Let me relate my story to you.....
My fish started dying just after Christmas. All my neon tetras, all my glolight tetras, ALL my platies inclusing 5 fry. I had NO idea why. Ammo, nitrate and nitrite were OK. I had not added anything to the tank. Food was the same, dechlor was the same, water changes were the same, on a Sunday.
I decided to move my remaining fish (a whiteskirt tetra and a few bnose fry with their father) to another tank and dismantle the tank. I was convinced it was possessed, I mean, all those deaths over a 2 week period. Random symptoms such as grey patches, swim bladdder problems. Taking the hood off the tank I realised there was a dog toy sitting in the AC300 HOB filter. Significance? When we treat the dog for fleas we also treat his soft toys. The offending toy had been thrown on top of the tank and had fallen into the filter. I didn't know this, I would have removed it if I knew.
SO, basically all my fish died from Frontline poisoning. The 5 platy fry dying was the worst as was losing the only male fry that survived to adulthood from my 2 fave red platies (which had died some time earlier)....
Sorry for the long story, but I guess I wanted to show people that no matter how careful you are or how you think everything is "perfect" in your tank that there may be something such as this causing the deaths.
I hope your fish are OK.