Low KH and pH Systems
Running a system intentionally kept acidic has its challenges. Systems kept at a pH of 6.0 or below cannot maintain a nitrogen cycle with the chemolithoautotroph species common in systems above 6.0. The lack of KH and pH is inhospitable for nitrifying beneficial bacteria in most aquariums.
In low pH (below 6.0) systems, the ammonia the fish excrete through their gills is quickly converted to ammonium and becomes non-toxic for fish, but at 10 ppm or higher, it can inhibit plant calcium and copper uptake by plants. Since there is little or no ammonia for nitrifying bacteria (above 6.0 nitrifiers) to process, the nitrite remains at 0 ppm.
If the system has a mature nitrogen cycle that was started with a pH above 6.0, slowly dropping the pH below 6.0 will initially cause a positive ammonia/ammonium test result. Maintaining a pH below 6.0 requires a shift in the microbe species responsible for the nitrogen cycle. It can take up to a month at 80° F (27° C) for the population of microbes responsible for the nitrogen cycle in low pH systems to mature. Once the new microbe population has matured the ammonia/ammonium test result will show 0 ppm.
Low pH systems are not an area with much scientific research at the time of this writing. The bacteria and archaea that maintain the nitrogen cycle at a pH above 6.0 are well studied, but below 6.0 is not.
If you start with a new low pH system, expect the time it takes to establish a mature nitrogen cycle to be two months or more.
Water changes should be done with water that has been pre-conditioned (KH removed and filtered through peat), so it is acidic before adding it to the system. Hobbyists that maintain low KH systems use a reverse osmosis tap water filtration system to create very low GH and KH water. Pre-conditioning is often done in a large plastic trash can. Sphagnum peat moss is commonly used to lower the pH of water in collection containers.
If at some point, hobbyists want to raise the pH to the basic (alkaline) side, they should expect to have to go through the normal time it takes to develop a mature nitrogen cycle. Any ammonium in the aquarium water can be quickly converted to ammonia when doing water changes with basic (alkaline) water. If the pH is at or above 7 after a water change, an ammonia test should be done to verify the water is still safe for fish.