Trying to grow coralline: Is testing for magnesium necessary in a FOWLR?

[julianp]

Hey all,

Hopefully this is not a stupid question, but I've never kept any corals so I am having a hard time understanding the alk/calcium and magnesium relationship.

I love the way lots of coralline algae looks covering the back walls and rocks of reef tanks and would like to begin dosing two part to encourage coralline growth in my FOWLR.

So far I have an Alk/KH test kit and the B-Ionic two part bottles. Before I begin dosing, I am going to buy a Salifert Calcium kit.

I know that lots of calcium usage by corals can lower magnesium, but would this be a problem in a FOWLR tank with growing coralline, but no corals as long as I were to keep up with small water changes?

I guess what I'm asking is, if I can keep my calcium up and it's not refusing to rise regardless of dose, is it really necessary to test for magnesium? I really would like to avoid the $30 cost of a mag. test kit if at all possible.

Thanks a lot,
Julian

[Sk8r]

If you're trying to grow coralline, yes, testing for mg is good. In people it functions as a laxative---so I wouldn't od it.

But if you can get alk and cal to stay up and they don't sink like a lead weight or refuse to rise at all, you have an average presence of mg. The chemistry of it is this: in good sea water your alkalinity s/b 8.3-9.3. Your calcium s/b 400-450. To attain this, you 'set' your mg by dosing to reach 3x the desired calcium reading. Calcium will not rise higher than the mg level allows it---so you could almost calculate what your mg might be by multiplying your calcium reading x3. I'm not sure that would work---ask Chemistry forum; but seems it might be a decent indicator.

If you are trying to grow coralline, one way to do it is have mg around 1500 and calcium around 480...which is kind of an overdose, and I don't like it that high, on general principles. But if you want coralline all over your glass, that is one way to make it happen---growing it on your rock, now, seems to be another matter. For one thing, some rock really readily grows coralline; but other rocks can't seem to---maybe because of what's on the surface.
HTH.

[julianp]

Great, thanks for the repy.

I ordered a Salifert calcium test kit and I'll try to make due with that and the alk kit I have.

If I still have little coralline growth in a few months I'll pick up a mg test kit. Somewhere between one third and 1/2 of the surface of my live rock is covered entirely in purple (was in someone's 3 year old reef tank before my FOWLR) so the rock I have should be able to grow it pretty well.