[Peter Finke]

Jordy, nice to meet you here, so finally it has worked.
Ypur questions:
1. tank size: Paros need not much space for their behaviour. Your tank is fine but think how you would raise the young. Some pairs tolerate the young, but you will really be unable to feed them properly; it’s too big. But you can try.
2. Filtration is not really necessary. Peat is best.
3. Substrate is not really necessary because you can’t use many plants in such water (no calcium, pH below 6.5, best between 5 and 6). At any rate the substrate must only cover the glass bottom. Leaves are important (oak, beech). Collect them dry from the wood and soak them in hot water.
4. The material of the cave is unimportant (plastic, clay, wood). But the cave should be small (diameter not more than 3 cm, perhaps less) and the surface level should make sure that no nest or eggs slide out.