I'm pretty sure the problem is not the brand of the fish food, but rather something being off in your tank.
How big is it and how many fish do you have left? Was it 70 liters with two comets and a loach or do I mix things up?

Problem here is that your tank is most likely (or for sure if it's 70 liters..) too small to safely accommodate these fish. It's possible that the comets bullied the fantails to death as well, since long-bodied goldfish aren't really compatible with fancies. It could also be a sudden ammonia spike in case you don't take good care of the water quality i.e. changing at least 50% of the water weekly, vacuuming the gravel and rinsing the filters in old tank water. You could also have insufficient filtration... so many things could be wrong, and only way for us to help, is you answering the questions and testing the water's ammonia, nitrite, nitrate and pH.