My wife had 6 Molly's in a tank and over a few weeks all but one young fish had just started shaking and died. When the last young fish was left I removed it and put it in a small tank with two gold fish. In a few days the shaking slowed and stopped, during this time it was eating the gold fish food Wardley Goldfish Flake Food. I put a small pinch of the Wardley Tropical Fish Flake food my wife had been using and the shaking returned. Over the next two weeks I fed it the goldfish food and all was well and 3 more times tried a pinch of the tropical flakes and the shaking returned with use of the tropical flakes. Has any one experienced a problem with their store bought food before?