150gr mixed shellfish or shrimp
100gr cod fish
150gr peas
1 garlic clove
½ squash
½ carrot
1 teaspoon Kelp
1 teaspoon Spirulina
1½ teaspoon Agar (ideally more)

EDIT: I have since experimented some more with this, and found out that agar works best when it is boiled. Simply boil it in some water, before adding to the blend. Also use more Agar than described here *

Chop the carrot and squash into rough pieces and boil for 5 minutes. You don’t want to boil them but just make them tender. Add the peas and boil a few minutes more. I mostly did this because they were frozen and I wanted them to thaw. If you have a powerful blender, you may not even need this step, as your fish can eat the ingredients raw. I just needed them to soften for my smoothie blender to cope.

Add the vegetable, fish (in chunks) and shrimp into the blender. Add 1½ dl of water, as well as the Kelp and Spirulina powder. Blend until you have a lean mix. You may need to add more water if it won’t blend properly. I used some of the water that I boiled the vegetable in.

Pour the mix into a pot and heat it up, without boiling. You don’t want to cook the mix, but simply dissolve the Agar powder into it. I used 1½ teaspoons this time, but I will use 2 or 2½ next time, to get a more firm result. Stir for 3-4 minutes and let it cool for 10 minutes (this may vary if you use more Agar, so keep an eye on it).

Before it settles, pour into zip lock bags. Make a rolling movement, to get rid of all the air in the bag, before closing it. You can decide the thickness of the final plates, but don’t make them too thick, because they will be harder to break into chunks once frozen.

Let the bags cool, until he mixture settles and solidifies into a gel type of texture. Put the bags in the freezer and place them flat until they freeze.

https://danishgoldfish.com/101/feeding/you-can-make-your-own-fish-food/