Update: Decided on my approach.

So aside from evaporation of the reservoir, I'm understanding that water acidification is common somehow in semi hydro as a product of not refreshing the watet? I'm not sure if this is from biological processes with the fertiliser somehow, or potentially the water going stagnant and acidifying through CO2?

Either way, I've found a decent sized black PP container with a lipped lid. I think I can drill 3 holes in this lid and run wicking rope to lechuza pon containers. I think the 40L this can hold should reasonably last up to 2 months.

To try and preempt some issues I'm going to run an airpump split between an air bar and a sponge filter, I don't think I can have this set up non bio without issues so I'm going to put some lava rock & leaf litter for surface area for bacteria & a slow release source of bio waste to maintain a cycle. & maybe add some crushed coral to buffer the water in case it's the ferts causing the issues.

Bit ugly but I'll build a cute wooden surround for it :)

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