In a swimming pool, what makes the water hard?

calcium and magnesium. these two contaminants produce what is commonly referred to as "hardness" in water.

No. Swimming pool water will not damage your teeth if the levels are in the proper ranges set forth by the NSPI. Yes chlorine is a dangerous chemical in the wrong forms or the wrong doses, but that is not the levels found in pool water if it is treated properly. The articles below listed by another reader are based off of Chlorine gas and levels way above what should be remotely found in any pool. The range for swimming pool sanitation is 2ppm - 4ppm of chlorine. Than means you have two - four particles of chlorine for every 1 million particles of water. Chlorine is used by nearly every water treatment plant in the USA to clean and sanitize our drinking water. At any given time, there is a chlorine in your water. (Depending on the time of the year and the source water condition the levels may be between .2 ppm and .5 ppm.) I personally have tested tap water where I have seen levels as high as 1.0 ppm. So unless your pool is extremely over chlorinated you have no worries regarding your teeth, and if it is you have more serious issues to worry about like vomiting, diarrhea, chemically burning your insides and possibly death. You should never swim in a pool where the levels exceed 10 ppm

I don't think hardness is something you feel on your skin. As long as your chlorine is high enough, you can be a little low on calcium. The reason higher calcium content ("hardness" refers mainly to calcium but also to magnesium content) is allowable in pools is because you're using calcium hypochlorite as your chlorine source (the pellets). The pool needs to be slightly alkaline to keep the chlorine from evaporating away. Ordinarily a pool will be buffered with carbonate/bicarbonate as well. Your pool guy probably checked for iron too but it doesn't sound like you're having a problem. Sounds like a healthy pool.

The best thing you can do is take a sample of the water in your pool and the pool's spec, then take it to the nearest pool stores like "Pinch a Penny" or etc to test the water in your pool. After they test it they will give you the results and recommendations on how to improve the water. You can also ask them questions and etc about anything pool related.

Surface Tension...(Sorry...small joke)

It sounds fine. How many grains of hardness are there? Your pH is fine and everything else is in line. You don't want your pool water as soft as you home water.

I think zinc might. check it out on google or something to make sure.