mussels and shrimps contain Thiamanese and can cause a vitamin defficiency in your fish so only feed them occassionally. Cockles do not and so are a safe staple. You can buy frozen food at many aquarium retailers but they do prefer larger more meatier treats than say tubifex which is so small it is just mush, just defrost before feeding.
I feed mine daily but skip the odd day, I probably feed around 6 times a week but don't stick to a strict regime.