Chemistry is a mess. Please help.

[duncangweller]

If you do a 20% water change it won't raise your alk to 8.6, It will only raise it by a few points.

From my experience if you can get magnesium to the right level, then the other two should follow pretty quick.

As far as dosing, get your tank to the right parameters, then wait a few days and test again. This way you can see what gets used up by your tank. Then dose back up to the right level and calculate what your daily use would be from how much it dropped over the few days. (Hope that makes sense).

The BRS calculator is just a guide, so don't take it as gospel, but its a great place to start. Your tank may also use more calc than alk or vice versa, so through your testing you will see what gets used and what you need to replace.

Once you get it set life gets a lot easier, but through adding corals and such you will need to adjust the amount you dose to keep up with the new demand.

There is a tonne of information online regarding this. Keep an eye out for articles from randy holmes-farley as he seems to be the reef science guy (they can be a bit heavy so maybe get a beer or a glass of wine before settling down to read it)

Hope this helps,
Dunc

[CM125]

Just remember 20 gallons is only about 20% of your system. The difference wont be a shock and a 20% water change is always a good idea to start!

[ponokareefer]

From what I understand, if your alk is low, you shouldn't be dosing calcium, you should be dosing alk. If your magnesium is at 1500, that is a little high, but not terrible. I definitely wouldn't be trying to boost it. With that much magnesium, you should be able to get your alk back up. What is your calcium level at?

Also, what is in your system for corals and inverts(such as clams)?

What is the magnesium level and calcium level of the new salt water?

What is the PH of your tank water and the new salt water you mixed? Alk can have a bearing on the PH. Here is a link to alkinity and PH. http://www.advancedaquarist.com/2002/5/chemistry