Instant ocean salt parameter

[mikellini]

I'd say it's worth raising mag to at least 1250ppm. Here's a snippet from Randy H-F's article on optimal parameters, regarding magnesium:

"Magnesium

Magnesium's primary importance is its interaction with the calcium and alkalinity balance in reef aquaria. Seawater and reef aquarium water are always supersaturated with calcium carbonate. That is, the solution's calcium and carbonate levels exceed the amount that the water can hold at equilibrium. How can that be? Magnesium is a big part of the answer. Whenever calcium carbonate begins to precipitate, magnesium binds to the growing surface of the calcium carbonate crystals. The magnesium effectively clogs the growing crystal surface so that they no longer look like calcium carbonate, making it unable to attract more calcium and carbonate, so the precipitation stops. Without the magnesium, the abiotic (nonbiological) precipitation of calcium carbonate would likely increase enough to prohibit the maintenance of calcium and alkalinity at natural levels.

For this reason, I suggest targeting the natural seawater concentration of magnesium: ~1285 ppm. For practical purposes, 1250-1350 ppm is fine, and levels slightly outside that range (1250-1400 ppm) are also likely acceptable. Higher levels may be fine, but there is no reason to keep it higher, with the possible exception of trying to kill bryopsis with certain magnesium supplements (which may work due to an impurity rather than the magnesium itself). I would not suggest raising magnesium by more than 100 ppm per day under normal conditions, in case the magnesium supplement contains any toxic impurities. If you need to raise it by several hundred ppm, spreading the addition over several days will allow you to more accurately reach the target concentration, and might possibly allow the aquarium to handle any impurities that the supplement contains (such as ammonia or trace metals).

An aquarium's corals and coralline algae can deplete magnesium by incorporating it into their growing calcium carbonate skeletons. Many methods of supplementing calcium and alkalinity may not deliver enough magnesium to maintain it at a normal level. Settled limewater (kalkwasser), for example, is quite deficient in magnesium relative to a coral skeleton. Consequently, magnesium should be measured occasionally, particularly if the aquarium's calcium and alkalinity levels seem difficult to maintain. Aquaria with excessive abiotic precipitation of calcium carbonate on objects such as heaters and pumps might suffer from low magnesium levels (along with high pH, calcium, and alkalinity). In general, magnesium is usually depleted at roughly 10% of the rate of calcium depletion, or less, depending on the creatures in the aquarium. Any depletion rate that is much higher than that is either due to testing errors, or water changes with a mix that has a different magnesium level than the aquarium.

Many people never need any magnesium supplements. Some salt mixes start so high that it will never drop below natural levels, and some calcium and alkalinity supplement methods, such as a good quality two part system, add enough magnesium that it should not decline."