Fiberglass Pool Chemistry

Unless you have waterline tile or piece of equipment that mandates a minimum CH, you generally don't need to add calcium. The gelcoat doesn't have plaster properties, so any calcium is simply a small preventative measure to help against certain types of potential staining. The TFP recommended levels are just fine.

You can test it, but I suspect the chalkiness you are experiencing is probably not calcium. Some might classify it as related to oxidation, or perhaps just the effects of the gelcoat reacting to chlorinated water. It doesn't happen to all FG pools, but it is quite common. Below is my experience.

There is a VERY large range of CH in most all pools. If you are not having issues, your CH is likely fine,

This forum has seen successful pool water management at CH levels of 1000+ and 50 ppm but that doesn't mean you should target those extremes. A CH from 200-500 seems to be very acceptable

Always test the CH of your fill water to better understand how it is affecting the pool.

Capr said:

just don’t understand how no one’s numbers seem to align and there’s no consistency across the industry.

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The industry once recommended fiberglass to need calcium similar to plaster pools. Whether the manufacturing process changed or most realized it didn't matter, you'll still see both the old and new recommendations out in the wild.

Chances are if one has a 1980s or older fiberglass pool, its been refinished with modern gel coat that definitely doesnt need calcium.

For us specifically the #s range high for the large parts of the county with high CH fill water who struggle to keep it under 500. They'd all need to spend thousands on a water softening system to max at 120 CH.


Also, +1 with Pat above. Yours sounds like chalking, not calcium scale.

Capr said:

I’m just confused why TFP and others recommend such a high level of calcium if it’s a known issue.

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There's two things about that comment we should address. First, there's an assumption being made that Orenda and River Pools have confirmed (scientifically) that calcium is the reason for chalkiness. I'm not claiming to know it all, but I've not heard of that, and it doesn't make sense really since "some" calcium in the water can serve to buffer the pool from certain products, like metals for example. The second thing is that TFP doesn't necessarily want you to have an elevated CH level unless it's warranted for plaster or equipment. Our levels start at about 250 and go up from there to a maximum allowable number. It's a very wide range designed to not only accommodate pool owners across the country and their various equipment, but are still ranges that are very manageable to the average homeowner.

So if your CH was say 200 and you had no heater CH requirement or tile waterline, you would be fine. By the same token if you had extremely hard water like in AZ with a CH of 800 or more, your water is still very manageable. The chalkiness appears in gelcoats across the country and in our TFP survey, there is no indication that the chalkiness is related to an elevated CH at all. In fact, as the case in my story, I tried managing the pool with a CH below 100 and the chalkiness continued.

As I said earlier, aside from someone who really let their chemical levels get WAY out of whack leading to accelerated oxidation (high pH and high FC), many gelcoats seem more prone to chalkiness simply from the effects of chlorine. Just an unfortunate chemical reaction.