I suggest there are a few things to sort out here. The first is, just what sort of tank is involved? "Walstad" method can vary a lot. What is the substrate? Soil has serious issues, but sand is very different. A sponge filter was mentioned as possible, I would add this. True Walstad method has very minimal water changes (the very low stocking levels allow for this, sort of) but this is never something I would remotely recommend.
The water is RO so that's fine, there are a number of suitable very soft water fish species that are "nano" sized. The substrate will affect some of these.
General comment on stocking. Sites like Aqadvisor cannot possibly take account all the factors involved, so at best they are a very rough guide. Each species of fish has needs that must be met or they will be stressed and prone to more problems. Some of these needs include numbers of the species for shoaling species, and there is the swimming activity level which may require a longer tank than some other species. The normal mature size needs to be considered--this is where the Widow Tetra fails; at 2 to 2.5 inches and requiring a group of at least 10, and being moderately active swimming, it is not suited for a 15g tank or a 20g.
The water is RO so that's fine, there are a number of suitable very soft water fish species that are "nano" sized. The substrate will affect some of these.
General comment on stocking. Sites like Aqadvisor cannot possibly take account all the factors involved, so at best they are a very rough guide. Each species of fish has needs that must be met or they will be stressed and prone to more problems. Some of these needs include numbers of the species for shoaling species, and there is the swimming activity level which may require a longer tank than some other species. The normal mature size needs to be considered--this is where the Widow Tetra fails; at 2 to 2.5 inches and requiring a group of at least 10, and being moderately active swimming, it is not suited for a 15g tank or a 20g.