Saltwater aquarium hobbyists have to worry about more water parameters than their freshwater brethren. One parameter of concern is pH. A low pH can be a problem in and of itself. However, it can also indicate other problems with the water chemistry.
Low pH Problems
In the wild, ocean water is extremely stable in terms of water chemistry. So most marine organisms are adapted to water with consistent parameters. This means that fish and inverts from the ocean have a hard time adapting to a pH lower than 8.2. Additionally, these organisms adapt poorly to rapid changes in pH since they rarely encounter such a shift in the wild. Either a low pH or a sudden shift can seriously stress marine fish and corals, and possibly kill them.
Ammonia
A low or decreasing pH can also indicate other problems in the aquarium. Ammonia can decrease the pH of an aquarium. While ammonia itself is a weak base, the biological processes that produce it release enough hydrogen ions as byproducts to drop the pH. Ammonia is toxic to living things in general and marine organisms in particular. If you find your water has a low pH, you should also check for an elevated level of ammonia. Any detectable level of ammonia is harmful to saltwater fish and invertebrates.
Calcium
A low pH can also warn of decreasing levels of calcium in the aquarium. Many marine invertebrates, especially coral and clams, will absorb calcium from the water column to build their support structures. Since calcium increases pH, the process of pulling it out of the water can drop the pH. So if you see a low pH, you should also check your calcium levels. If you keep marine invertebrates, you need to test the calcium levels monthly. Additionally, you need to supplement the tank with extra calcium to make up for the calcium they suck out of the water. Keep the levels between 380 and 450 mg/L.
Excessive Carbon Dioxide
Another possible cause of low pH in the aquarium is an increase in carbon dioxide. Since most marine aquariums have plenty of water movement, especially reef aquariums, this is usually not the case. However, if your pumps and powerheads break down and the water's not moving around, this could happen. Extra carbon dioxide in the water will make it more acidic. You can counteract this by increasing water movement with powerheads and other pumps that move the water around.
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