Thijs, “soil” is not exact a concept that is helpful here. Walstads “soil” is a nutrient-rich soil for the normal aquarium with normal fish and normal water. There are Paro-aquarists successful with pure peat-soil, but I am not in favour with this practice. One cannot fight the destruction of peat bogs in Asia by destroying our European ones. Better use a pure quartz bottom, but this only will help in anchoring your plants that need this; it will not nourish them. In most of my Paro-tanks there is a bottom-layer of some millimeters up to one cm only, with leaves on most of the places upon it.
There is no serious nitrogen-problem with a well built Paro-tank, even of 12 liters only and containg one pair, since the fish are very small, eat little amounts of live food. The live food is essential; you cannot overfeed your fish. In tanks of 30 liters small (!) groups of Paros can live very well, even in company with Boraras spec. But mind: The Boraras need more food than the Paros even although they are still smaller. They swim around and need energy. Ans they prevent young Paros from growing up. I never had Paros in tanks bigger than 50 or 100 liters maximum; you cannot watch the full behaviour, and it’s a nonsense from the desire-of-space-fish-point of view; a Paro pair needs a cave and only little space around. In nature its home range is not bigger than a small aquarium. You should not keep too many fish in a tank if you intend to see the full behaviour.
Mind that they are heavily endangered fish; quite different to Boraras or Trichopsis. We don’t “keep” Paros, we care for them and see them fulfill the full life-cycle including the whole breeding program. If you leave the young in the parent’s tank, you have the advantage that feeding Artemia and Moina serves the old and the somewhat grown young; only for the first two weeks you often need still smaller food. But if the tank is decorated with Javamoss, floating Ceratopteris and leaves on the ground, there is enough hiding space for 10 to 30 young even in a 12-liter-tank. Later on this tank is too small and feeding will produce too many relics; you surely have to change the water more often.
Your remarks on the water purifying powers of some riparian plants with the main amount of their leaves above the water surface (as Spatiphyllum for instance) are mainly applyable to the normal aquarium with tap-water and not the blackwater-tank with nearly destilled water. They need a thick layer of soil for their big roots. Partly applicable is leading the air-roots of Monstera and similar plants into a tank without such a bottom, but a purely blackwater tank has much too pure water in order to nourish that plants. Nevertheles, you might experiment with this. There, you could surely care for “easy” species (as P. linkei or P. nagyi or P. paludicola, the latter living at the borderline of black and clear waters). But most of the interesting Paro-species (as the true P. deissneri, the bintan-variants, opallios or ornaticauda) need the full program of attention, the best water and the least possible germ concentration. Mind that P. ornaticauda for instance normally have clutches of ten or 15 eggs only.
The germ concentration is a very important point; it plays no role in Walstad’s book since most “ornamental” fish are able to stand high concentrations. The most useful measuring kit for Paro-aquarists wpuld be a kit measuring the amount of germs. These kits exist and are sold by a highly specified industry at moderate prices, but up to now it is not sold in the hobby shops since normal aquarists don’t need them. If they used them, they would be frightened and startled about the values quite common in nice looking brightly illuminated plant-rich tanks with a normal fish community exhibiting no signs of illness; but that is not the Paro world. The more plants requiring nourishment you try to keep in a Paro tank, the less it is an optimum for you fish. Remember: they live in small slightly floating creeks in peat bogs of the primeval forest with high concentrations of humic substances.
As the temperature is concerned: I do not use any heaters in a room never colder that 21 degrees Celsius. Sometimes it’s 24 degrees. It should never exceed 29 degrees. Colder that 20 degrees is possible for quite long a time but not half of the year. In course if time the fish will become weaker and more prone to diseases.