If I am doing 20% water changes every two weeks using natural seawater, should I be worried about magnesium? I plan to get a tester. I never thought much about it, but when discussing my newly acquired sps frags (2) the question of magnesium came up.

with NSW, i would not worry to much about mag, unless your calcium and alkalinity get out of whack.

if you do decide to test and dose it ... it should be between 1350ppm and 1450ppm in my opinion.

thanx! calcium and alk are good, I think I will get a kit just so that I can check from time to time to get a feel for how much gets depleted. BTW, I was just reading up on magnesium, the first link that came up discussed dosing epsome salts. Is this crazy? I wonder how accurate measuring would be.

i know a few people that use that to bring up mag levels. there are no ill effects that i know of. you will find that testing it is like doing a lab test !! lol

I have used epsom salts before. The only possible drawback is that it adds sulfate as well, but as long as you keep up with your water changes it wouldn't pose a problem. Measuring doesn't have to be all that acurate, because it takes a ton to increase mag just a little. IMO, it's worth testing every now and again just to be sure it's where it needs to be since it's tied so closely to ca and alk.

I like the liquid Mg supplements (ESV's additive or Kent Tech M if I run out of ESV). Epsom salts are a pain to measure out, imo. You should be keeping your Mg in check regardless of what supplement you use to do it.