I found a product that is far superior: It is sold by Aquarium ecosystems (
www.aquaticeco.com) and it is called something like gel-food. It is a coarse powder like gelatin that acts as binder but produces a better paste. It is very rich in protein, and in other goodies (minerals in particular). If anybody is interested, I have the breakdown of its composition, just not with me right now. It holds powders (such as vitamins or spirulina) very well. It seems to have good flavor, because the fish go for it very well. It is not expensive. Finally, gelatin "melts" very quickly on contact with the tank's temperature, whereas this gel is more stable. All in all, I like a lot better than gelatin, which is of course almost completely devoid of nutritional value.