Sodium bisulfate is an acidic salt that can be used to reduce the PH of fish tanks…I was using it in my prior setup. I was using live baby brine shrimp…about 1/8th teaspoon every other day for a maximum of eight paroes. PH and conductivity stayed in the 4’s and below 40 respectively, and I was doing 40% water changes every week (this tank had the unusual attribute of having the conductivity drop over the course of the week). For tannins, the substrate was made of peat moss and I put decaffeinated tea bags to seep into the new water before water changes.
Anyhow, I have decided to alter my plan slightly. The tank will have a layer of peat moss on the bottom (I bought the peat moss before I remembered how destructive its harvest is 🙁 may as well use it now that I have it), capped by a layer of sand and with plants and leaves above that. May boost water changes to 3 times a week. Also, may add a second tank to my paro setup…if I spot fry I would be able to move the parents quickly and maximize fry production.