Did my tank suddenly "mature"

[pseudorand]

My 120g is just under a year old. Before getting corals, I zeroed in my 2-part and other dosing. I had alk stable at 8.0 with 0.3 ml BRS soda ash per day and no Ca dosing because it was at 450 and never changed. Mg was at 1500 and also never changed. Nitrate was consistently <5 but >0. Phosphate was 0.3 for a while, but I was slowly bringing it down with phosguard and weekly water changes. Iodine was at 0.06. Iron was at 0, but I couldn't seem to bring it up. Parameters had been stable like that for at least 45 days because I refused to put corals in my QT until I knew (thought) my DT was stable. I have an ATO and salinity is always 1.025.

Then I changed everything:
* I added a skimmer
* I bought a big bottle of Brightwell Ferrion and started dosing 15 ml/day
* I added 9 frags that just completed QT. Lights down for a week, then back up.

The next time I tested, Alk had fallen to 7.4 and Ca to 350 and Mg to 1200. I brought these back up to 8.5/400/1350 and recalibrated my dosing. I'm now at 1 ml/day soda ash to stay at about 8.5 dkh. I'm also dosing 1 ml/day CaCl2 and Ca has fallen to 370 (still trying to get the stable dose right). I haven't had to dose Mg again and it's still at 1350

Stranger still is that I'd been doing weekly 15% water changes and sucking away cyano in the process. My queue for a water change (in addition to it being Thursday) had been green algae on the glass, which always built up to quite visible in a week. But its now been about 3 weeks since my last water change and there's not a bit of green algae on the glass. Some harder build-up is occurring (maybe coraline algae, but it's white/grey, not pink/purple) though. And the cyano, which had been a persistent problem, started to build up on the rocks and gravel the first week but is now receding. In addition to neglecting water changes for 3 weeks, I haven't changed my phosguard, but phosphate stayed steady at 0.1.

I've reduced my iron dosing to 10 ml/day and it now seem stable at 0.15. I have a sizeable ball of chaeto in my sump, along with some pompom that's quite a bit less vigorous.

* Why did my Alk/Ca consumption jump so suddenly? (my 9 frags are quite small)
* What happened to the green algae on my glass (not that I want it back, but...)
* Dare I call my cyano problem solved? If I start weekly water changes, will it come back? (Yes, I use RO/DO)
* Does any of that sound like a problem?

 

[ichthyogeek]

I doubt your tank suddenly had a growth spurt.

How often do you do your testing? If you test on a rare basis (say, monthly), it wouldn't surprise me that 9 frags would suck up a great deal of magnesium, calcium, and carbonates (alkalinity) in that time period.

What type of frags do you have? If they're 9 fast growing SPS frags, then again I would not be surprised.

Were there any frags in the tank beforehand? I've definitely heard of corals having a "break in" period, where they wait a bit to fully acclimate to the tank, and then start sucking everything up as they realize that yes, they can grow without disturbance.

Algae wise, my guess is that the chaeto and pompom you have, ended up 1) outcompeting the other algae (+cyano) for nutrients once you added sufficient enough iron to the tank, and 2) potentially released allelopathic chemicals (most likely unharmful to corals) to inhibit the nuisance algae's growth.