Baffled by calcium test discrepancies
 

Hello all,

This past week I have been rather perplexed by water chemistry. I am pretty new to this and didn't realize how quickly calcium and alk levels could drop in a reef tank. Over the past two months with only doing water changes, my calcium went from 600ppm to 345ppm and the alk went from 11.7 to 7.8. pH has remained stable around 8.2-8.3 (depending on what the light in the room looks like when I look at the color swatch) and the SG has been rock steady at 1.024.

Needless to say I decided to get on top of my water chemistry - especially considering how much SPS I've been adding and started a slow process of bringing things back up to recommended levels. I bought the Elos magnesium test and a liquid magnesium supplement this week as well.

Here's where I'm confused. The tank is a 90 gallon + 20 gallon sump + fluval Fx5, so I figure after displacement from rock, sand and equipment I've probably got around 100 g total volume. I've been able to get very specific and accurate in the amounts of reef builder (seachem) that is required to raise the alk by 'x' amount and since tuesday have gotten my alk up to 10.6. The Elos magnesium test is great, with titrations that have end points as exact and obvious as anything from my university chemistry class and the supplement raises the Mg levels in a very exact manner, I went from an initial test of 1000 to 1250 over 4 days. However, my calcium levels are perplexing me. I have two tests, one which came in the Nutrafin master test kit (what gave me the initial result of 600ppm) and the supposedly more 'reliable' Instant Ocean calcium test. I believe it's a marineland product. Well, no matter what I do, or how much liquid calcium I add the Instant Ocean test shows a result of 360ppm. Like, dead steady. Even after adding the maximum dosage of calcium the bottle says you can add to a tank that size in a day. The Nutrafin test however, returns results anywhere from 100-120ppm higher and it seems more responsive to the dosing. I'm tempted to think that the nutrafin test is over-estimating the calcium concentration as there was no spontaneous precipitation at the 600ppm level and I never added anything to the water in the beginning than the instant ocean salt mix. Neither test is really good in terms of having a clear end-point, like light pink to light mauve with a gradual change from one to the other is pretty hard to tell. But in either case, the color always begins to obviously change at the same point, no matter how much calcium I've added.

Since they return such dramatically different results I'm starting to wonder if it's the tests. Has anyone ever experienced this? alk and magnesium levels that adjust readily and politely but calcium that just won't budge? Is there a better, more obvious and definitive calcium test kit out there that has a stronger and faster color change than pink to darker pink/blue? Because it's possible that I'm just really really bad at reading them too...

I seriously doubt your Cal was ever anywhere near 600. With you mag and Alk being that low it most likely would have been snowing in your tank. How heavy is your Cal demand? ie. how many sps colonies in your tank? You are using a Caddilac mag test kit but a hyundai cal kit, toss it and get either a Salifert or Elos as they dont go by colour gradient but by a rather definate endpoint.

awesome, thanks. I will swing by Ocean City on my way home from work and pick one up. I've got 4 reasonable sized acro colonies, a couple smaller acro frags, a smallish superman montipora colony and a bigger purple montipora. I also have some sort of thick, glow in the dark green shelf style colony that has a really thick skeleton (girl at big al's didn't know what it was, but it's wicked), a brand new australian purple pink chalice frag, a trumpet coral colony that glows in the dark with 5 major branches and about 10 sub branches, a branching frogspawn that has been growing like a weed (it's started with three heads in November and two of them are in the process of splitting so it will have 5 shortly) and an aussie elegance coral that has put on about a half inch of width since november. When I type it out it sounds like I have more than I thought I did.

If I hadn't already blown my entire life's budget I'd buy a calc reactor and be done with it... but you can only make so many 500 dollar purchases at a time.

You can dose DIY 2 part or use Kalk to maintain your tank at a much more reasonable price than buying a reactor or buying premade stuff.

Stay away from the diluted bottled calcium supplements, they're a waste of money. I'm willing to bet that you could dump the entire thing into your tank and it would hardly budge your calcium level. Calcium Chloride is a much more cost effective way of raising calcium and is what is used in two part supplementing schemes along with Sodium Bicarbonate (baking soda) for alkalinity.

So I went out and bought the Elos KH and calcium tests to go along with their magnesium test and wow... the other test kits were way off the mark. the Elos kit, with it's REALLY specific and obvious end points showed a calcium of 410 and a dKH of 13. I thought my dKH was low and in fact it's high. Glad I didn't add any carbonate today! Now as soon as my lab grade calcium chloride arrives in the mail I'm set :biggrin: