Kent Marine pH control minus 250g
Directions:

First read the considerations for use section below. This is a professional strength product. Exercise care! If in doubt, use less, then measure pH. Consult an authoritative guide for the proper pH for the type of fish you are keeping. Add 1/8 (one-eighth) teaspoon granules to a pint (1/2 liter) of tank water and mix well. Disperse this amount around the tank for every 20 gallons (80 liters) of tap or tank water to lower pH. Several additions will be required to overcome highly alkaline waters and pH rebound will be experienced. PH stabilization takes 24 hours. If pH is still too high, repeat process, increasing dosage, if necessary. Each addition will reduce alkalinity until pH becomes manageable.
Considerations for use:

FRESHWATER AQUARIUMS: If you are using Reverse Osmosis water or if your tap water is very soft (low total or general hardness, i.e. GH), use Kent R/O Right first to increase total dissolved solids to the proper value. Measure the total alkalinity (buffering capacity or KH) of your water with an alkalinity test kit. (Warning: use of this product on soft waters of low alkalinity may cause a pH crash and kill your fish!) If the alkalinity is low (see chart below), add Kent pH STABLE to raise it to the proper level. If the alkalinity is high, use this product, as directed, to slowly bring it down. Next measure the pH. If it continues to be high, repeat addition of this product, very slowly. Make pH changes gradually, never change pH more than .2 pH per day! The best method is to make alkalinity and pH changes outside the aquarium, then do a water change. This is always less stressful to the fish.