Most marine organisms are adapted to very stable, very consistent water chemistry. For this reason, you must keep their water pH high and avoid a declining pH. The pH can be raised several ways, but each method has pros and cons.
Raise and buffer the pH passively by selecting aquarium decorations that leach calcium and biocarbonates into the aquarium water. This includes calcium-rich rocks, like tuffa rocks, and substrates, like coral sand. These material slowly and steadily release calcium into the water. This method of raising the pH has the advantage of costing little. However, because it works slowly and steadily it may not keep up with certain biological processes that drop the pH. For example, many corals and other invertebrates will absorb calcium from the water column to build their exoskeleton. Tanks with such organisms need more aggressive measures to maintain the pH and calcium levels.
Various supplements and chemical buffers also can be used to raise pH. Many are cheap and easy to dose. However, some may raise the pH too quickly. Most aquatic organisms adapt poorly to sudden shifts in pH, even when the changes are to a more favorable pH. Additionally, some products, like kalkwasser, require you to pre-dilute them or add them to a sump instead of the main aquarium, making them more complicated. However, many supplements that up the pH have the added benefit of adding calcium to the water, which improves the health of many saltwater invertebrates.
Calcium reactors are designed to add calcium to the water rather than up the pH per se. However, adding minerals, like calcium, to water tends to buffer the water, and make it harder for the pH to drop. The downside to calcium reactors is that they are expensive and tricky to set up. However, DIY models tip these pros and cons; homemade reactors are even harder to set up, but cost less. If your tank includes calcium-hungry invertebrates, a calcium reactor might be a good investment to keep the pH stable and maintain calcium levels.
Maintain pH and avoid sudden shifts with good aquarium practices. Many harmful biological processes tend to lower the pH, so controlling them can prevent pH drops and raise pH. Overfeeding and overstocking an aquarium can lead to high ammonia levels, which lower the pH of an aquarium. So, keeping up on your water changes, and avoiding overfeeding will prevent ammonia buildup, thus keeping the pH high and avoiding a declining pH.
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