Just a comment on the pygmy cory issue, though you may have gone past it now. But the issue is still relevant.
Cories are shoaling fish that need a group. We try to have "minimum" numbers because there are times when space may be that restricted and the minimum may be all that one can manage for this or that species. But the more fish of a shoaling species there are, the healthier they will be, and less stressed because of the numbers. The dwarf species of cory, like Corydoras pygmaeus, needs a group of at least 7-8 and that is very minimum; I would consider 9+ better. So this aspect has to be remembered when one is trying to work out numbers and space. Three or four pygmy cories would not be very happy; 8-9 would thrive more or less.
While I'm here...you could have a single male Betta in this tank of 30 liters (roughly 8 gallons). Betta are not community fish, so adding any other fish species is not advisable.
Other options would be a group of one of the "nano" type fish, like Ember Tetra, or one of the lovely dwarf rasbora species in thee genus Boraras. And here a group of 8-10 pygmy cories would work. Most "nano" fish will be wild caught, so water parameters are important. You mention starting with RO...is there a reason your tap water is not OK on its own? Having to prepare water for every water change or emergency can be something to consider, as life is much easier and simpler if the tap water will work.
Byron.
