Lots of things will influence the size fish tank you use. What kind of fish, how many fish, size of media beds. If you're growing fish to eat then you probably want to be able to feed them as much as you can for fast growth. In that case you'd need fewer fish or more filtration/beds. If you're not eating the fish then you can regulate plant nutrient by the amount of food you give the fish. Check out Bright Agrotech on YouTube. Dr. Nate has an abundance of information for understanding all that stuff. But I would recommend start small and add on (fish). I didn't and now I have overnutrient issues.