Hi Jordy,
when my setup is all new (new sand, fresh wood and leaves; no water or filter-sponge from running tanks), I usually try to wait 1 week, until I put the paros into the aquarium.
But I think, this can’t be generalized. In case of doubt try to wait 2 weeks. Over time you will get an eye for the moment the milieu is right.
A small bag of peat-granules helps to keep the pH low.
My first paro-aquarium was running almost one year, before I got the first fish. 🙂
Greetings,
Martin