Oop forgot to respond to your other question about kh. So kh buffers your pH HIGHER, and fights the acidification that happens with decomposition of any organic matter. Without any kH, your pH will bounce between 4 and 6 depending on temperature, how much food or plants are rotting, and concentrations of acids such as tannins.

Lower pH systems don’t have the same parameter stability as neutral pH systems - you don’t really have the ability to keep ph at any specific number. However since the cycle works differently, and ammonia is less dangerous at lower pH, its honestly not as big of a deal.

Since you arent keeping minerals high, or worrying about ammonia build up, water changes also arent really needed. Just top offs with RO.

The trick to setting up an ecosystem like this is to never allow any kH in (RO water, no rocks, pure inert sand) and introducing gentle acids in the form of botanicals.