First, for any discussion or help, we need to know if you are changing the hardness of the water when you do changes. pH and tannin levels are secondary to mineral content when it comes to Apistogramma. My tap is soft and acidic, and I could keep any Apistogramma, and breed most of them if I wanted to. When I was getting a lot of blackwater Apistos and trying to breed them in my old house, where the water was moderately hard and alkaline, I had to use RO and rainwater/snow melt, and there I had to be calculating with every weekly water change. It all depends on the water going in.
Tannins are almost decorative, although they can be used to trigger spawning. You may not have fish breeding on your agenda...
If your choice of species is limited by what the fishfarms produce, I would agree with
@Colin_T , as long as you weren't breeding them. The industry has weeded out the difficult species. If you aren't in Australia like him, and can get fish from the wild (only a very few Apistos are captive produced) then you need to really know and understand your water. So which species of Apisto is important. Apisto cacatuoides or borellii can handle most water, while bitaeniata or njisseni will have shortened lives and no breeding in moderately hard water. For a fish farmer, the species only needs to reach adult size and get to market. What happens after that is of no concern to them. Lifespan matters to us though.
So your question draws questions. What is your tapwater/source water like? Which species has your eye?
