Guppies dying one by one

Hi, I have a 55 gallon aquarium with about 10 guppies, 5 guppy fry (in a breeder box), 2 Cockatoo Dwarf Cichlids, and a small pleco. I feed spirulina brine shrimp and bloodworms. Recently, two of my guppies, and a Gardneri Killifish have died, all in the same way. They go to the surface and stay there for most of the day. Then I find them caught in the filter intake, still alive. I turn off the filter, and they swim away. Eventually, they just stop swimming, and float around in the current. Then, they die. I don't know what's happening. My fry - which should be the most sensitive - are fine. My parameters are fine, too (pH 7.2, Ammonia 0, Nitrite 0, Nitrate 2.5, Temp 78). Since they stay at the surface before they die, I thought it might be an aeration problem. Nope, I've got an airstone in front of the outflow. All of the healthy fish are swimming all over (even to the lowest parts of the tank), so I don' think the problem is aeration. Does anyone know what the problem might be? Any help is appreciated!! Oh, and the they have vivid coloration all the way through. They die with all of their bright colors - it's weird.