Help! Low Ca & Alk

Mix it with RO water and pour it in the overflow.

 

Or you could pour it in your overflow dry, if your tank is drilled. That's what I do. It will dissolve slowly, and not become waterborne while granular, which is the important part.

 

I mix it with water.. and slow pour it into my wet/dry main sponge/filter area.. so it gets diluted with some sump water before its pumped into tank.

 

Just wanted to clarify that epsom salt is not a suitable long term magnesium supplement. I use it as well as a lot of others, but just be aware water changes must be done regularly to mitigate the sulfate levels. Epsom salt is magnesium sulfate.... only about 8% of epsom salt is actually magnesium. Over time sulfate levels will rise to undesirable levels if you don't do regular water changes and if you have to raise mag a large amount it's probably better you get a better form of mag like magnesium chloride.

If you scroll down about half way there is a good read on this particular subject:
Do-It-Yourself Magnesium Supplements for the Reef Aquarium by Randy Holmes-Farley - Reefkeeping.com

You can get magnesium chloride at bulkreefsupply.com