Very interesting set up! Long ago I built a solar heated chicken coop using the same principle. The thermal storage was 5 gal black metal containers stacked inside a south-facing plexiglass exterior wall.
You may want some fish for a purely practical reason: mosquito control. Depending on water temperature gambusia or small goldfish would work. Or you could use BT wafers, a.k.a, mosquito dunks.
If the soil and fertilizer in the water lily pots isn't enough, you could use almost any type of terrestrial plant fertilizer that you already have. Start with very small doses and increase until you see the desired growth on the floaters. If you have a major algae bloom, do a big water change and reduce the dose of fertilizer.