That looks awesome, I wish I had full sun like that. I got my solar-powered fountain pump in the mail today, and my indirect sun just creates an intermittent bubble every 5 secs, lol sad.
I wouldn't worry about TDS with the rain water and the established tub. Bettas are hardy, and I doubt you're polluting your water at all if they're just feeding off live larvae. Also there's probably snails and inverts breaking down particles in the water, and nematodes liquefying waste in the soil. If you're curious you can grab a pretty accurate meter for $20.
You have any insect problems in the garden? Everything looks healthy, what's your secret? Ladybugs?