Magensium Low?

[DeepBrew]

Hi all, I apologize in advance for the long post. I have a 75G tank which has been running since 10/2015. It started out as a FOWLR, crashed in 11/2015 and was fallow until the beginning of March. While it was fallow I decided to make it into a reef, built a new 29G sump and added a skimmer. During this time I also slowly raised the salinity from 1.020 to 1.025 and started checking magnesium with a Red Sea test kit. The tank is still lightly loaded so I've not had any trouble keeping calcium and alkalinity up but magnesium has always tested low. I use Sea Chem Reef Salt. Since I've installed the skimmer Ammonia, Nitrite, Nitrate and Phosphate always measure zero with API test kits.

I also use API tests for Calcium and Alkalinity. These are the tank parameters today:

Ca=420, kH=9, Mg=1160, pH=8.3, SG=1.024

I made up a gallon of new water, let it sit overnight and tested to see how it compares to the tank. I used 1 gallon of RO/DI water and 5oz of Sea Chem Reef Salt, these are the results:

Ca=400, kH=9, Mg=1040, pH=8.4, SG=1.024

I should note that I also just got a bottle of 35ppt reference solution to calibrate my refractometer. I believe that's why my SG is a bit lower than I normally see. I'll take care of that over the next few days.

I'm going to start using BRS 2 part to maintain CA, kH and Mg but I'm not sure I trust the Mg test results I'm getting. I've done a fair bit of Google work but haven't found others saying their Sea Chem Reef Salt is low in Mg. I also haven't found people complaining that their Red Sea Mg test is reading low, most seem to complain that it reads high.

Do you think I should trust the test and add Mg to the tank or would you recommend trying a different test before I raise the Mg level?

Everything in the tank seems happy and healthy so I'm a little concerned that I could be trying to fix a problem that isn't really there.

Thanks in advance for your help!

 

[downbeach]

I would get your SG up to 1.026 first, and then take those readings. Seachem Reef Mg, should be reading around 1400ppm. Are you sure you're doing the Red Sea Mg test properly, I know it can be a little tricky with having to mix each of the 5 drops of reagent A for 15 seconds between each drop. Here's a demo, in case you haven't seen it yet.

https://www.youtube.com/watch?v=8NSGs6j-iXg

I would also suggest that you get a test kit with better resolution, i.e. Red Sea Pro, Salifert, etc., if your going to start dosing. Make sure you thoroughly mix your salt mix before using it to ensure there wasn't any elemental stratification that may have occurred during shipping.

 

[bertoni]

We do see bad batches of salt products, but we also see a lot of testing issues. I'd get a second opinion before dosing much. That's very low, and I suspect a problem somewhere, but I don't know where.

 

[DeepBrew]

I watched that video twice before I did the water tests yesterday. I'm doing it properly as best I can tell.

I'll add some salt to the gallon I made up, get it to 35 ppt and test again. I may order a Salifert test kit as well.

What's the best way to mix salt in a bucket?

 

[Steve175]

Salifert Mg is cheap and reliable and I would confirm with latter before dosing. It takes a lot of Mg solution to raise your level by that much if the level really is that low.

 

[downbeach]

I watched that video twice before I did the water tests yesterday. I'm doing it properly as best I can tell.

I'll add some salt to the gallon I made up, get it to 35 ppt and test again. I may order a Salifert test kit as well.

What's the best way to mix salt in a bucket?

Here is a good practice for mixing your ASW:

http://successfulreefkeeping.com/learn/reef-tank-101/salinity-mixing-saltwater/

 

[DeepBrew]

Thanks for the link downbeach, there's good information there. I worded my question poorly though. I'm really asking for tips on stirring up the salt mix in the bucket to make sure it's not stratified. Is rolling the bucket around sufficient or is something like a drill with a paint stirrer needed?

I've ordered a Salifert Mg test kit and will not dose Mg to the tank until I have results from it too.

Thanks again for your help!

 

[tmz]

I haven't found a loss of homogeneity to be an issue over the years .So, I don't routinely stir the dry mix before using. I suppose it can happen in some circumstances and would take considerable effort to remix if it does. Periodic testing of newly mixed salt water is a way to tell whether or not the ion concentrations are what you expect. FWIW I prefer the Salifert tests for magnesium and calcium. For alkalinity I use the hanah checker and cross check with the Salifert test occasionally.

 

[DeepBrew]

I adjusted the gallon of test water to a salinity of 35 ppt by adding 0.5oz of additional Sea Chem Reef Salt this morning and tested after it's rested for about 10 hours. These are the new results:

1 gallon RO/DI water
5.5 oz Sea Chem Reef Salt
Salinity = 35 ppt
pH = 8.4
Ca = 440
kH = 10
Mg = 1200

The magnesium is still testing low with the Red Sea kit but better than before. So part of the problem disappeared when I calibrated the refractometer using a 35 ppt solution and adjusted the salinty of the water accordingly.

I have a Salifert Mg test kit on the way, it should be here early next week. Until then I'll use 35ppt make up water on the tank to gently bring the salinity up. I'll post an update when I have results from the Salifert test.

 

[bertoni]

1200 ppm is fine. The canonical ocean average is about 1275 ppm. I wouldn't worry much, but I probably would dose it up a bit out of obsessiveness.

 

[DeepBrew]

Thanks bertoni!

 

[bertoni]

You're welcome.

 

[DeepBrew]

Update
I now have the Salifert Mg test kit and have raised the tank salinity to 35 ppt. Here are today's test results:

Salinity 35 ppt
PH 8.3
KH 9
Ca 420
Mg Red Sea 1240
Mg Salifert 1140

So the Salifert test read a little lower than the Red Sea. Which is likely to be the more accurate? Should I just call it about 1200 and raise it from there? Or just stick with one test or the other since they're fairly close?

Thanks!

 

[bertoni]

I'm not sure which might be more accurate, if either. I would assume the magnesium level is somewhere around 1200 ppm. I might raise it a bit, but even 1140 ppm is safe enough in my experience.

 

[DeepBrew]

Thanks! I'm not going to obsess about it but I'll definitely keep an eye on it. I'm not having any trouble keeping the calcium up and everything in the tank seems happy so I don't want to mess around with it if I don't need to.

 

[tmz]

Probably ok at even 1140 ppm but I'd personally bump it up by 150 ppm( to 1290 / 1350 ppm) in increments of 50 ppm per day That level or even as high as 1450 ppm won't hurt anything and may make precipitation of calcium carbonate less likely. It's a personal choice and relates to some extent on what types of organisms are in the tank.

 

[DeepBrew]

I may do that. I have some BRS Mg salts mixed up. Their calculator says I need to add about 32 oz. If I add 8 oz a day as part of the make up water for 4 days that should do the trick.

Thanks again!

 

[tmz]

You are welcome.