The most valuable result of filtration is the water movement. Parosphromenus don’t live in stagnant but in slowly running waters. I always recommend a small bubble air driven filter because of that and as an additional security measure for small tanks; my Paro-tanks are 10 liters each. In any case, you must learn very careful feeding to run these tanks without any filtration. But it is possible. I have no filter at all in these tanks since decades. The purifying effect of filtration is much overestimated in the aquarium hobby; it’s a big business in the first rate. The only thing which deserves more importance is the movement of the water. But the fish can be kept and bred in tanks with non-moving waters and they develop normally and healthy.
Nevertheless: I always recommend to beginners to use a small bubble filter. It’s additional safety, although it’s not necessary. If Ceratopteris, Riccia, Vesicularia and some other plants grow in that soft, acid water, a pair of Paros and their small offspring don’t need filtration and water movement. Nevertheless: use it, if possible, but don’t overestimate it.