Chemical imbalances

While I may be way off, it's my understanding that the water can only hold so much alk, ca and mag {along with the other essential/trace elements}. So if you have high mag, for example, the water will hold less ca and/or alk. If you have high ca and mag both, the alk will be lower and thus not as available to the corals...maybe not enough so that it interferes with growth (not just size either...), lower pH, etc. There is also the Redfield Ratio that some adhere to/believe. You can 'google' Redfield Ratio if you'd like (Reef Central also has some info/comments on it).


As far as figuring out what may have caused the mag to drop, mag is used in the tank by several things (corals, coralline algae, etc). It just isn't used as quickly as CA and ALk. That's why at some point, some have to dose Mag. Also, as David said, the salt mix may have been deficient/too low in Magnesium for what the tank needed. Testing the newly made salt water, especially before adding it to the tank, can be a very good idea as the water you add may be much higher in CA and Alk, than what's in the tank and therefore could possibly stress/shock the inhabitants...depends on levels, size of tank, amount of SW being added, etc.


Others may be able to explain it better than I have. /DesktopModules/ActiveForums/themes/_default/emoticons/blink.gif

 

That's not jgh for alk. I try to keep it between 9 and 11

 

While this is true with many things, saltwater is a bit more complex. Keeping higher Mg will allow the water to contain more Ca and alk. Seems strange, but true! High Mg interferes with the crystallization of calcium carbonate allowing saltwater to be "supersaturated". This article explains better than I can:

http://reefkeeping.com/issues/2006-07/rhf/

We are very lucky to have a world class chemist like Randy in this hobby!!

My Alk is generally around 7. I know ULNS folk like to keep it around there as SPS seem to burn much above 9 in a ultra low system just FYI.

Remember that stony corals use alk and calcium to make their skeleton, not just Ca, so that does alter the water chemistry quite a bit and is why lots of folks dose Ca and alk together. (I like limewater) I would advise caution however, while you need to dose if your system falls below the accepted minimum values, I see many people that end up chasing numbers for no reason. They just keep messing with the system and do more harm than good. Your levels are a bit low so dosing is warranted, but always start by brining the Mg up before messing with Ca and alk for the reasons in the article.