If your water is truly R/O water and not just softened with a water softener then you could mix your water from outside with the water from inside depending on why you're using R/O water in the first place. If the water is just hard and maybe has a high pH that would work fine and you wouldn't have to spend extra money on something like R/O Right or equalibrium. If the water is possibly contaminated by heavy metals or pesticides, etc. just stick with your water from inside with the R/O right added. As for your tetra, is it a black phantom? I wonder if it could possibly be a bala shark. Are you positive that it's a type of tetra? I think someone said it already but if you're on well water and it's R/O water, then don't waste money on a dechlorinator.