Depends on the fish you have really. If you have vegetarian fish then you are gonna want to feed them things high in plant materials, i.e. veggetables...but if they are carnivorous, you will want to feed them more meaty foods, i.e. beef heart, shrimp. You can mix the foods with out too much problem...but remember the fishes needs and normal diet. Also, the plant eating fish have a different intestinal system than meat eating fish...so each type is more adapt to digesting their ideal food source. What i like to do is go to the grocery store and get a bunch of things. Here is a list of the stuff i mix for my fish and how i make it(note there isn't any amounts...you just have to learn to wing it and figure out what works best:
Garlic
Celery
Brocili
Brusselsprouts
Peas
Banana
Avacado(very little)
Beef Heart
Liver
Chicken(thigh or breast)
Shrimp(cocktail normally)
Fish(salmon, trout, cod, tuna...doesn't really mater...i like to use cod if i can)
Octopus OR Squid(depends on my mood...lol)
Vitamin B pills(the gel cap liquid ones)
I place these all in a blender so that there is an average ratio between them so there isn't one dominant food source. I then put in about 1/4 water and squeeze four Vitamin B pills into the the blender. Then i puree it for about 2 minutes or until it is a uniform size throughout. Then i tank an eye dropper or serving spoon and pour it into an empty Formula One fish food package( i use this cuz there are already neat little cubes there that used to hold the food)and then i place it in the freezer. When i want to feed them i take out one of the cubes, thaw it in some warm water and give it to them in the tank. The nice thing about it is that it never has the same stuff in it so there is plenty of variety and its allowing all species of fish to get an opportunity to eat.
Hope this helped! Enjoy!