Generally your water being white is going to be a bacteria bloom, and bacteria blooms are not bad, rather they are GOOD because your tank needs that bacteria to stay on track chemically. I think you probably had more than one thing going on here. Doing large water changes and changing filters sounds like a recipe for a bloom. If your water turns white, it is not something to panic about. The other part of it though is that Cardinal tetras really are infamous for being sensitive fish. Did you check your temperature? Did your water flow change at all? Gasping can be from reduced oxygen in the water and that can be caused by a raise in the temperature of the water or less surface agitation.
There are a fair amount of things that will cause fish to die, and unfortunately when you lose one, more are bound to be lost because of stress and changes that you make to the tank following the deaths. I doubt that your food would have caused it...but if it is a year old its probably not too nutritious for them anyway so splurge and get them some good new stuff eh?