also contrary to a lot of beliefs about varying pH with fish depending on what i was doing with them the pH could have been as low as 4.0 or as high as 7.6. where the stress on the fish comes about is with the polution in the water. if the water is clean you can move them around from high to low or low to high pH without causing any harm to them. on a closed system i would NEVER vary the water's ph quickly. the reason for the pH changes could be to stimulate spawning by lowering the pH or to raising the pH to put the fish in a breeding rest cycle. a pH of 4 would be to stop bacteria or viral infections without meds. the plants were not removed from the aquarium regardless of pH. the water could have been pure RO with a rejection rate of 99.75% to full tap water 400ppm hardness with total alkalinity of 150ppm and a ph of 7.6. i only use GAC to remove chlormines in the hatchery. because of disease control, i always used lab nitrifiying bacteria to innoculate any system. i stopped using salt for any purpose when it proved basically worthless. (this of course would not be the case with fish that are accustomed to brackish water or with a high alkiline water such as africans from the rift river system, or several species of livebearers. if you want beautiful sailfin mollies go to saltin sea in so. CA.
excuse me for rambling, but there is seldom a one word answer.