@Kidsandliz if you have the correct balance of fish and plants, you won’t need a filter or to add plant fertilizer.
I saw a presentation by some urban gardeners that had an immense setup with hundreds of feet of horizontal PVC pipe and an aquarium. They had drilled a hole every foot along the pipes. They put a seed into a holder in each of those hundred holes. The water was pumped from the aquarium up to the start of the pipes. The water flowed along the bottom of the pipes over the roots of each plant. After the last plant it went back to the fish tank.
To keep it in balance, they had a pound of fish for some 3 digit number of pounds of plants.
With this size planter, it would probably support a single fish less than a inch long. You need food for the fish and light for the plants.
@hamjudo I don’t know much about keeping fish. We had goldfish for about 3 or 4 weeks as my kid was insisting she wanted fish. Insisted for weeks so I finally caved when it appeared not to be a passing fancy. Goldfish are incredibly social, interacted with us (which was pretty interesting) and the cats (they were also cat TV) and as I found out need about 5 gallons for each one which we did not start out with and can live 20 years which is, no doubt, why they are so smart. It turns out fish are more work than cats. Fortunately my kid decided she didn’t want fish ofter all and so we gave them away to another family.