Discus are very particular about pH. Keep your pH below 7 and above 5.5. The ideal pH for discus is 6. At pH levels above 7, discus are stressed. Below 5.5, the pH is inclined to plunge rapidly, so I find 6 to be comfortable for both the fish and the fishkeeper. Alkalinity and pH are closely related. Hard water naturally tends to be alkaline. Soft water naturally tends to be acidic. This is because of the buffering capacity. Buffering capacity represents the presence of alkalinity (carbonate hardness) and the ability of the water to maintain high pH. It is a chemical balancing act. Just enough carbonate hardness and the pH remains at the desired level, too much carbonate hardness and the pH will remain high, too little carbonate hardness and the pH will crash. Maintain your carbonate hardness at around 10 or 15 dH and you should have no problems with pH. Check your pH with every water change until you are able to get a feel for how your water behaves. If you notice that the pH drops quickly, you must add back carbonate. If your pH resists change to lower values, you must remove carbonate. There are many methods of lowering your pH, most with some form of phosphoric acid, from drops to powders, but one of the gentlest and safest methods is through the use of peat moss. Because the peat adsorbs carbonates and acidifies the water, you should be able to maintain desirable pH and carbonate levels through the use of peat alone.
