The use of sodium bicarbonate for the maintenance of the proper hydrogenion concentration and bicarbonate content in marine aquaria using a closed circulation, was recommended by Breder and Howley (1). It was pointed out by them that this substance was more suitable for such purposes than quicklime which is used in the Plymouth Aquarium, because the latter “disproportionately increases the calcium content.” Atkins (2) disagrees with this opinion chiefly on the grounds that sulphates from metabolized food do not tend to increase the acidity of the water. Atkins quotes Smith (3) as stating that the urinary SO4 comes from ingested sea-water, and neglects entirely the fact that Smith had reference to the bulk of the urinary salts. As Smith remarks on page 494, a fraction of the urinary S04 is of metabolic origin, and this fraction is of course the only S04 which is significant in the problem discussed by Breder and Howley.