Since this is something I do frequently I thought I'd add my 2 cents.
This isn't an exact recipe as I do it differently every time, but basically I use any veggie that I may have that I don't want to eat anymore or is a bit to old (not green and fuzzy tho) or was cheap.
Zucchini
Carrot
Spinach (can be the frozen stuff)
Squash
Peas
Anything really
Unflavored gelatin
**Optional**
Sultanas
Calcium
Raw shrimp
I use a food processor to mush the peas a little, don't make it too much of a paste, it's better if it retains some shape.
Spinach and all leafy greens if fresh are best done this way too. Or if you want you can cut cut them up by hand.
**Sultanas should be cut in half, the outside is not usually eaten so if left whole may take snails days to get to the good stuff.
The shrimp should be done in the food processor like the peas**
The zucchini, carrot, squash anything like that I grate, I'm too lazy to do it by hand so I use the grater in my food processor.
All I do is wash and cut off the very ends, then grate the rest.
Once all the veggies are done I mix them together and put them in a microwave safe bowl (rice cooker works well) and nuke it for about 4mins.
If a lot of liquid is in the bottom I drain it into a cup.
To this I add some boiled water and add the unflavored gelatin.
Mix until the gelatin is dissolved but stir gently you don't want lots of bubbles.
Add this to your veggies.
I fold this through gently to avoid bubbles the more bubbles in the mix the longer it will float in the tank, then I spoon the mix into ice cube trays.
I press the veggies down into each cube with the spoon so the veggies are packed. Then I'll add a bit of the liquid just so the gaps are filled between the veggies and the liquid is level with the top of that cube.
I put these into the fridge for a couple of hours then into the freezer.
Once frozen, I empty them out and into a container (with lid) and store it in the freezer.
I then just take what I want out each feed and put them straight in the tank.
They'll float for a short time then sink to the bottom still holding they're shape.
Snails love it and my bristle nose eat them too.