Do Acros use a lot of magnesium?
 

I have been dosing for a couple of years now and a few months ago I added about 8 acros so they are started to grow now. I have only ever really dosed alk and ca, but now I am suddenly having to dose magnesium every day. The only thing that has changed are the acros so just wondering if they use a lot of magnesium.

How big are these corals? They would need to be rather large to drop Mg daily.

No they are not huge. 3 frags are about 3 inches tall and the rest are smaller. I have been adding 20 mls a day and that is only taking my magnesium up to 1380. I have recently had a little bit of cyano on the sand and now the power heads. Would that use up the magnesium?

Adding a few acro frags shouldn't affect chemistry at all over days (or weeks really).
Mg can bind with other things, I'd guess, but no, not cyano. OR that I've ever read.
"only" 1380 is more than fine for Mg, not sure what you're trying to reach.

But measure it, test again 24hr later and see the difference, if any. Keep in mind error in hobbyist test kits can be significant.

Yes, I have been testing every 24 hours and it is dropping to around 1300. It was 1200 last week. I realized it because my alk was dropping and I was having a hard time getting it to go up. Just was a little curious to why I suddenly would need to start dosing mag every day. A mystery I guess!!

Can't say, I'm not someone that uses Mg. But I can say it's not some frags. Maybe others that dose it can chime it.

I dose it as part of balling salts. But if I'm not mistaken Coraline will readily use it

I'm curious how you're dosing for alkalinity. If you're dosing sodium carbonate instead of sodium bicarbonate it could be precipitating out with calcium carbonate.

When I had a mature SPS reef I went through quite a bit of Mg and I auto dosed it. With only a few corals as long as you're testing and adjusting your water change water you shouldn't need to dose much. Instant Ocean has been ridiculously low in Mg lately.

I would consider 1300 to be the ideal level for Mg, but I don't think 1200 would hurt your corals at all. Some people successfully keep a reef tank without even testing for it.