My recommendation would be that utilizing your current substrate by itself with the addition of root tabs under any heavy feeders would be fine. I use PFS and have a densely planted low tech tank. I would certainly avoid CO2 (as this will lower your Ph) and there is still a bit of researching to be done as to gymnothorax polyuranodon being a species that does not need occasional salinity/marine conditions. In any case of perceived health issues I would increase the salinity to at least lower end brackish and if maintaining it in freshwater, I would watch that the PH maintains itself toward the higher range (8.0 and higher) with a healthy TDS. Being from a hard water area, I can personally attest to seeing brackish species (Monos, scats, archers) doing terrific in very hard fresh water.