UV sterilizer and Mg levels

[Stephen Keen]

I've got a 440 gallon tank, 75 gallon sump, and 40 gallon fuge, Deltec skimmer, BRS phosphate reactor, carbon reactor, and a Sterilight UV sterilizer that is commercial grade capable of 5 GPM which turns over the water in my tank in just over an hour (360 gallons of actual water).Lots of small sps, plenty of fish, plenty of zoas, 500 lbs of LR. My levels are:

KH stable at 10-10.5
PH is always a little low but stable at 7.9-8.0, been like this for 2 years
Ca stable around 400-450
Strontium at 12 ppm (very close to natural seawater)
Potasium around 390 ppm (very close to natural seawater)
Salinity stable 1.026
Temp very stable at 78-79
Mg...ALL OVER THE PLACE! 1050-1400
Water changes every 2 weeks with coralife salt

I dose everything above and watch my parameters very closely...it helps that it set in front of my desk.

My question is this...can the UV light effect my Mg levels? I know at a glance most people would say no but it might possibly be that. I deal with hydroponic gardening for a living and am very in touch with the industry. It has been realized that in hydroponic gardening using a uv sterilizer can break down nutrients used to feed the plants, in Australia you can actually by an additive that keeps the bonds from breaking down making the nutrient viable for a longer amount of time. Plants need calcium and magnesium just like our reef tanks. Plants were getting Mg and Ca deficiencies from using the UV sterilizer. I was told that the bonds of the elements are broken the they fall out of the solution. My theory is that the same thing is happening in my aquarium. I dose Mg like you wouldn't believe, I buy BRS Mg sulfate and chloride and mix together. Even with regular water changes it is very hard for me to maintain my Mg levels, I can dose it to 1500 and within a week the levels can drop, and do drop to 1050 and would stay at 1050. I added zeovit mag to my calcium reactor recently but I still need to dose.

Its possible that others running UV may not have noticed this because either most people do not check Mg or they are not running a commercial grade UV sterilizer that sterilizes their tank almost every hour.

I'm looking for an answer from someone that truly understands the binding and breakdown of elements in our tank. I can guess all day long myself but I know that someone is out there that can shed some light on this subject. I do plan to turn off the UV sterilizer and run an experiment myself...but currently I have a little ich on a few new tangs so I don't want to turn it off at the moment. It may be a month before I even consider doing so.

Any thoughts? Anyone else having Mg problems besides myself? Is there another answer to my problem?

 

[Randy Holmes-Farley]

No, there is no possible way the magnesium drops 1500 to 1050 ppm in a week except with huge water changes with a low magnesium mix. There is just nowhere for all that magnesium to go. We are talking about many pounds of magnesium in your tank, not a trace element.

The UV won't impact magnesium. It mi9ght deplete a bit of calcium and alkalinity and a very tiny, tiny bit of magnesium if the inside is getting coated with calcium carbonate.

Things that will are water changes with a low magnesium salt, and a slow decline with calcification by coralline algae and corals.

What salt mix are you using? What magnesium kit?

 

[tmz]

The Coralife you are using is high in magnesium. I run a system packed with growing sps and don't need to dose any mag at all to maintain mag around 1500ppm as I use this salt for waterchanges.

 

[Randy Holmes-Farley]

Sorry, I missed the Coralife comment. Assuming it is not a bad batch, there must be testing error.

To lose 450 ppm of magnesium in a week in that size tank represents a loss of 765 grams of magnesium ion. The normal route for magnesium loss, making calcium carbonate with some magnesium incorporation, would require the precipitation of a huge amount of calcium carbonate (roughly 19 kg), dropping calcium by more than 4000 ppm and alkalinity by more than 560 dKH. That seems unlikely to have happened.

 

[Stephen Keen]

I use salifert test kits bought at different times. It actually isn't a drop from 1500 in a week after thinking more about it, it would take a bit longer than that. But it will drop from 1250-1350 to 1050 in a week or so without a water change. If I don't dose and just do water changes my Mg is never above 1050, I'm positive about that. I'll get some more accurate reading this week and should be able to give you a daily usage amount, since I'm using Mg supplement in my Ca reactor it won't be a true daily usage though unless I were to turn off the reactor which I won't do. At least I could get a more accurate estimate than I have now. I had a 300 gallon system on another location that I moved and put in a new tank (the tank I have now) and the old system had the same problem.

Currently I'm using approximately 16 lbs a month, you are right....pounds. My Ca reactor is on 12hrs a day at full stream with the co2 at about 2 bubbles per second, so I'm adding quite a bit of calcium too. On top of that I also dose Ca, about 200 ml of calcium chloride per week.

 

[Randy Holmes-Farley]

OK, if you go through a lot of CaCO3 media that may be deficient in magnesium, then you may need to add some more dolomite to the reactor, or once a week or so add some magnesium to the tank.

FWIW, I'd measure some new slat mix to see how that comes out.

 

[Stephen Keen]

OK, if you go through a lot of CaCO3 media that may be deficient in magnesium, then you may need to add some more dolomite to the reactor, or once a week or so add some magnesium to the tank.

FWIW, I'd measure some new slat mix to see how that comes out.

That's what I already do. Newly mixed saltwater has 1500 ppm of Mg, tested it already.

I still have my suspicion that its the UV sterilizer. If it happens in hydroponics nutrient solution if can very well be happening in my tank. I'll run my own experiments and see what I find out. I'll put another powerful uv light that I have on small tank with nothing but saltwater and I'll take readings before and after a couple of weeks of running it.

 

[Randy Holmes-Farley]

I'm not sure what you are proposing happens in your tank with UV. Magnesium is not tied to organics in reef aquaria, although it might be in fertilizers or fresh water. UV is known to break metal ions out of organic complexes, and the heat can also precipitate calcium carbonate. In seawater, however, takes some well known forms with by far the majority as free Mg++ ion so I don't see how UV can do anything to that to make it more or less "available".

Here's the relevent section from one of my magnesium articles:

Magnesium is present in seawater as the Mg2+ ion, meaning that it carries two positive charges, just as calcium does. Most of the magnesium is present as the free ion, with only water molecules attached to it. It is estimated that each magnesium ion has approximately eight water molecules tightly bound to it. That is, water molecules that are so tightly bound that they move with it as the magnesium ion moves through the bulk of the water. For comparison, singly charged ions like sodium have only 3-4 tightly bound water molecules. A small portion (about 10%) of the magnesium is present as a soluble ion pair with sulfate (MgSO4), and much smaller portions are paired with bicarbonate (MgHCO3+), carbonate (MgCO3), fluoride (MgF+), borate (MgB(OH)4+), and hydroxide (MgOH+).

which is from:

Magnesium in Reef Aquaria
http://www.advancedaquarist.com/issues/oct2003/chem.htm

 

[Stephen Keen]

I don't have an explanation of how I think that the UV could be doing this. It is basically for me process of elimination and until I have a definitive answer yes or no I can't rule it out. I understand that with your explanation it shouldn't be possible but then again something unusual is going on. I know a few hobbyist that have smaller tanks than mine with more coral that don't even use half of the mag supplement that I have to, in fact they very seldom dose it at all. There is something that is effecting the levels besides just normal consumption...would you agree? Doesn't my usage seem unusual? I have no other explanations of how this is happening.

The hydroponic solution that I'm mention being effected is a salt based nutrient, nothing organic about it, no bacteria, nothing alive. What I was told about the hydro solution is that the UV breaks the bond that magnesium and calcium have and causes them to fall out of the nutrient solution. I've seen pics of the plants used with a UV, they look Mg deficient.

 

[bertoni]

That explanation of magnesium and calcium precipitation can't be correct. The metal ions are loosely associated with water molecules, and can precipitate only by combining with a carbonate ion, at least in significant amounts. UV won't affect that process.

 

[Randy Holmes-Farley]

FWIW, UV sterilizers get hot inside and are known to precipitate calcium carbonate inside of them, at least from seawater. If that same effect happens in fresh water, then perhaps it could be a way that the tiny amount of magnesium in fresh water could be depleted, but seawater has hugely more magnesium and not much alkalinity, so it can't be a way to deplete magnesium from seawater while it possibly could be from fresh water.