Here's what I used: the recipe is adapted from some of the others in the pinned topic.
1/2 can tuna
1/2 cup frozen peas
1/2 cup fresh green and yellow beans, washed and diced
1 medium carrot, peeled and diced
1/2 cup broccoli, washed and diced
1 red plum, washed and diced
1 clove garlic, peeled
1 tsp paprika
(I would have liked to use rice, but I didn't have any left
I blended the dickens out of everything in the food processor. I added the water from the tuna can, and about 1/4 cup of dechlorinated water, as well.
I used Knox gelatin; it says on the package that one envelope would set 2 cups of liquid or 1 1/4 cups solids. I used 2 packages of gelatin, and dissolved it in a cup of boiling, dechlorinated water. Then, I mixed it with the stinky, greenish-brown mixture from the food processor, lined a 9x13 pan with plastic wrap, and poured the whole mess in. Then I put it in the fridge.
Every 5 minutes or so I've been tapping the pan with a knife to release the bubbles.
If this recipe works out, and my fish like it, I think I'll try using different fruits and veggies next time!
I've gotta say, this is way cheaper than store-bought fish food! And it was fun!