Price is per kilogram.
Soda Ash is a supplement for dosing marine aquariums to increase Alkalinity levels when your pH is below 8.3.
Alkalinity is a complex measurement. It is not a "thing" in the water. It is actually the summation of many things, all of which together provide a particular chemical property. The reason that aquarists measure alkalinity is that in normal seawater, most of it consists of bicarbonate and carbonate. Bicarbonate (HCO3-) is what corals take up and process into carbonate (CO3--) in order to build their calcium carbonate skeletons. Consequently, alkalinity is an indication of whether or not adequate bicarbonate is present in the water.
Normal to high alkalinity implies adequate bicarbonate, while low alkalinity implies that it may be in short supply. In the absence of any method of supplementing alkalinity in a reef aquarium, the water can rapidly become depleted of bicarbonate. Alkalinity's depletion from normal to unacceptable levels can take only a day or two in some reef aquaria, although it can take longer in aquaria with a lower demand for carbonate. When the water's bicarbonate is depleted, corals that deposit calcium carbonate can become stressed and even die.
We recommend that reef aquarists maintain an alkalinity of 2.5-4 meq/L (7-11 dKH, 125-200 ppm calcium carbonate equivalents).