I soak in Epsom Salt often for relief of chronic pain. I know I can't get concentrations high enough in a pool to do any good for pain relief, but was wondering if I could add it to my pool water just to make it feel better. Would it effect my water balance in any way?

If it is okay to add, how much should or could I add to an 18-20k pool?

I'd just add regular Sodium Chloride. The effects on pool chemistry are known and benign, and it will make the water feel better.

Magnesium can be poisonous, and I doubt any of us know enough about it to determine what the safe level would be in pool water.

People sometimes add relatively small quantities of epsom salt to the pool to help make future calcium deposits easier to clean off. However the level of epsom salt normally used for therapeutic soaking would be corrosive to most metals in contact with the water (metal ladders, copper heat exchange coils in the heater, etc).

In reply to JohnT: Water can be posionous in high enough quantities. Actually our bodies need magnesium and most people have magnesium deficiencies. Magnesium Sulfate (Epsom Salt) is used for lots of things, including use as a compress or soak to reduce pain, promote healing and draw toxins out of the body.

I pour 4 lbs of it on top of me in a hot bath and soak in it for about an hour. No negative side effects at all. Just reduced pain. Now drinking it is a different matter. It's not toxic, but it will certainly help if you're a bit blocked up if you know what I mean.

I don't want sodium cloride in my pool. Neither of them are really good for you. Although our bodies need both, that is one thing most people are not deficient in!

So JasonLion, how much epsom salt can I add to 20k gals and still be under the corrosive limits?

I know I've seen the calculation before that showed at what ppm Magnesium Sulfate begins to be corrosive to pool equipment. I recall researching it years ago for my Jaccuzzi. The only thing I remember was that it was A LOT - I knew I would never put that much in my Jaccuzzi.

I have never heard of this being done in a pool, but for spa's some people add about 30 lbs of epsom salt for every 1,000 gallons of water. That should be safe for metals, though it might be a problem for natural stone work if water is splashing onto the stone and evaporating, leaving the epsom salt behind.

Great, thanks! That gives me something to go on. I know I wouldn't put near that concentration in my pool.

But soaking in a tub is different from swimming, both in the amount of time you are in the water, and the parts of the body exposed, e.g eyes and mouth. Soaking in water with epsom salts CAN lead to magnesium poisoning.

JohnT, are you sure you don't have magnesium confused with something else? Hypermagnesemia is very rare! Unless you have kidney problems or consume a lot of magnesium orally at one time, you shouldn't have a problem with magnesium. It would be even more rare to get magnesium poisoning by soaking in it - I'm not even sure that is possible due to the way magnesium is transported through body tissue versus the digestive system.

As for having the eyes and mouth exposed to the water, that wouldn't make a difference. The only way you would get too much magnesium is if you consumed a lot of it orally and had kidney problems. If you are healthy, your body gets rid of excess magnesium readily - you may have diarrhea, but you won't be "poisoned".

You may want to do some research on it. Magnesium Sulfate (Epsom Salt) is beneficial for many things.

This is equally true for salt. The fact that most Americans eat too much salt is completely unrelated to pools. Saying that having salt in your pool is unhealthy is just plain wrong.

High sulfate concentrations are destructive to cement products (well documented in the construction world) due to the high pressures that the sulfate minerals exert when they crystallize; this would translate directly to more rapid destruction of concrete or stone decking. Likely plaster too. Read this for more detail:
http://www.pwtag.org/TECHSulphate%20Attack.php

You are free to do whatever you like with your pool. We will not recommend things that we know will cause harm to infrastructure or equipment, nor are we in the business of promoting homeopathic remedies.

Thanks MITS! That's exactly the kind of info I was seeking. I don't want to add anything to my pool that will damage the plaster or the equipment. So, it sounds like I should be able to add a very small quantity without doing damage.

Some studies* indicate that high levels of magnesium could have an adverse effect on the cement in the plaster by converting calcium silicate hydrate to magnesium silicate hydrate and by reacting with calcium carbonate in the cement and in marble aggregate. In most cases, the magnesium concentration probably won't be high enough to have a strong reaction unless a lot of magnesium is added.

However, in the absence of any clear benefit, I think that magnesium and/or sulfate should not be intentionally added to a plaster pool.

Zodiac sells Zodiac Salt Cell Protector, which is magnesium sulfate. I think that the magnesium reduces the potential for calcium carbonate to precipitate. I don't know how much actual effect there is or why they use magnesium sulfate vs. magnesium chloride.

*Note: The studies were not in swimming pools, so they might not be applicable. Swimming pools would have a much lower concentration of magnesium, and any effect might be negligible. However, I don't think that adding magnesium, especially large amounts, is a good idea.