Ghia, to begin with your first sentence. The crypt species you mention are not “recommended” to stand the conditions of Paro-tanks, but – indeed – they need similar conditions. However, there is a very remarkable difference: They live in slowly s t r e a m i n g waters at special places only. At these places there are constant nutrient flows from the bottom; they do not exist on other places.
Up to now we are unable to cultivate these species under submersed conditions, that is: in an aquarium. If we cultivate them it is in an emersed way in a special soil, mainly composed from beech litter.
What does that mean? It means that in a tank with stagnant water it is impossible (up to now) to cultivate plants that require both: a stable very acid water near pure H2O, but a constant flow of special nutrients from their roots. If you move yor water (by a filter or by air) this is not the same; up to now we are incapable to imitate that nutrient flow and nevertheless keep the water surrounding the leaves with that extremely low content of minerals and that low pH (often 4 or less) that both is necessary for them. Therefore we try to cultivate such species under emersed conditions (and even that is a compromise).
To put it in other words: The structure of the aquarium is not suitable for the submersed cultivation of these crypt species (as pallidiveria, fusca etc). But remember: Our Paros do not occur on such places where they grow but on places mostly without any submersed plants. There maybe quite amounts of riparian grasses, wood, old leaves, but nearly no sumersed aquatic plants. Why? Because they cannot stand these extreme conditions.
What you are thinking about is an old dream of some aquarists: the underwater garden in a tank; but that is o u r dream, it’s not the reality of the habitats in nature.
I cannot recommend to you other crypts as suitable because they need other conditions than the Paros. Of course, you can try C. affinis, but it favours much harder waters less acid. Or you can use Sri Lanka-crypts (as wendtii or willisii) but it’s the same with them. They will live for some weeks, but kept under the conditions of our Paros they will eventually get weaker and die.
So, your idea – take those species that occur on places with very pure water and a very low pH – is allright in the first moment, but it is not if you think further: You cannot provide that nutrient flowing milieu for them that they need without destroying the stability of the Paro-milieu.
I shall not exclude that somebody manages to find a “third way”, but it wil be a compromise not ideal for the Paros and not ideal for those crypts. (By the way: In Europe, we have some crypt-specialists who cultivate some of those species, under emersed conditions). In the aquarium trade they are not to be found, and will not be in future, for the reasons mentioned. They are not suitable for cultivation under the conditions of aquarium tanks.