First I really love the appeal of a symbiotic relationship like that between the fish and garden systems. It is genuinely clever and that you've gotten going in the pursuit (putting it mildly - prototype four!) is encouraging.

I see that you've noted your location - North Carolina - which leads me to a question of sorts: Have you given consideration to more heat-saturated environments?

The reason I ask is because my area of Texas can get 100+ for consecutive days, and other markets might also deal with lots of sunlight causing weathering issues. I don't have a swimming pool but I do know several owners past/present who lament how their equipment needs frequent replacements due to the baking type effect. I'm hoping this is something you could consider in advance so that you can tailor your marketing or potentially innovate a structure using materials that will address customary failure points. Hoping for the best in your project!