measuring phosphates and magnesium

[Gyr]

I have the API combo kit for measuring water paramaters, and I am pretty happy with it. I have two questions about monitoring reef parameters:

1. If my KH and calcium are both consistantly on the upper end of desired range (KH = 11-12, Ca = 480 - 500), does that mean that magnesium levels are okay? My kit doesn't include a magnesium test and I had read in a few places that without adequate Mg levels, you can't have high-ish Ca and KH at the same time (they would precipitate out). If this is an incorrect assumption on my part and you feel it is worth getting an actual Mg test, which do you recommend?

2. I am on well water of pretty good quality (TDS of water straight from the tap befor it goes through my RO unit runs in 40 - 80 ppm range). According to the API test, the phosphates in my tank water have alway been undetectable (maybe they were high for a short time when I first cylced my tank--it was too many years ago for me to recall). I still have rare, mild outbreaks of brown algae even when nitrates are at 0 so I am wondering if the Phosphate test is accurate. Do others find the API test for phosphates reliable?
Thanks in advance,
Kurt

[Clownfish518]

Mg is another test completely separate from Alk and Ca. The Mg level does have a bearing on Alk, but only indirectly

PO4 tests are fairly accurate, but the home tests can only detect levels of inorganic PO4. The levels of organic PO4 can't be tested in a home test.

And PO4 is necessary. Corals need it for their zooxanthellae. The goal is to reach a balance in your tank, where what is created is consumed. When the balance is a little off, the opportunistic algaes and cyano make an appearance.

[Gyr]

Thanks, Clownfish.

So it sounds like adequate Mg levels can NOT be infered by combination of high-ish Calcium AND KH. If that is true, what Mg test do you guys recommend? I am using IO's Reef Crystals for salt mix and it sounds like that should have pretty good Mg supply.

I am stumped about Phosphates...If I can only test for inorganic phosphates, but there is no home test available for ORGANIC phosphates, should I just not sweat it if my API Phos. test keeps coming up 0?

[Clownfish518]

I like Salifert tests personally.

I guess my philosophy is I don't manage to the numbers. I watch my tank and if everyone is thriving, than I am good. If something looks off, I usually do a water change, and if it is still off, I look for the cause. The inhabitants tell me before a test can when something is off.

For example, I see a bit of cyano on a rock this morning, which means to me I have some PO4 in the system, but low nitrates. So I am going to do a water change and add some GFO.

[Gyr]

Cool.
I hope to someday be that in tune with my reef. I'll just keep plugging away, and try to keep learning...like looking up GFO in the abbreviations link