Both mom and I've been using this food, and it's a very healthy and good goldfish food
It contains mostly natural ingredients, and nothing that would be bad or unnecessary for your fish!
Need I mention that her fish are like two big fish balls? xD
Red belly is a sign of internal bacterial infections. Very hard to spot, and when the belly is finally red, it's impossible to save the fish. First symptoms (in my experience) can be a misshaped belly, rather lumpy and deformed, and a very lethargic fish. That's why the best thing to do now would be, if you have any fish left, to feed the rest with an anti-bacterial food to make sure it won't spread any further! It could be that the others already have it, so treatment is crucial!!
You should also, of course, test the water right away, post the results, and perform a 50-60% water change right away! If you want to, it could be good to add Melafix or something else to support the fish during treatment.
This is something horrible and very difficult to treat, so you can't wait with it