The best way I found to feed with Formula 2 was to take the package out in the morning and open it up and let it defrost all day on the counter. Then in the evening you take the package outside and find the favorite flower in your garden and pour it all out on the ground near the flower and let it fertilize your flower.
Honestly I never did like the stuff (can you tell) I mean you defrost it and then try and break it up, but the gel still kind of holds it all together and you try and break it up more and it just sits there still together laughing at you, kind of like the money you could have saved with Geico.
Anyway if you do want to stick it out, about half a cube should work for how many fish you have. Although if I remember right it is more veggie stuff and you might want to feed those fish some more meaty food. If you are going to ask "what food would I recommend" I will have to defer to other smarter people than me about that.
HTH
Scott