Fiberglass pools tend to stain very easily. To contol the stains, you have to keep your water balanced, and use a sequestering agent. I know that stains will usually disappear over the winter when the pool is closed. If you don't close your pool, then you need to get rid of the stains using ascorbic acid. While doing the stain treatment with ascorbic acid, it is very important to add the sequestering agent - something that says "controls metals". Once your pool is stain free and the sequestering agent is in the water, then the chemistry should be kept as follows:
Chlorine should be kept in the perameters of what your cya is.
PH should be kept between 7.2 - 7.4 (when the ph gets above 7.6, it can react with the chlorine and let any metals that are not sequestered fall out of suspension and stain the surface of the pool)
Alkalinity should be kept from 80 - 100
Calcium levels should be anywhere from 150 - 250. Some say that fiberglass pools don't need calcium, but I found that my pool reacts better with a clacium level of at least 150. If the calcium gets too high, it sometimes reacts to cause staining also - so anything under 250 is ususally good.
These are just my own observations of dealing with a fiberglass pool. I have my pool for eight years, and have really concentrated on what works best to keep it stain free. I have tried all kinds of chemisry levels - and this is what I have found to work the best for me.
I keep my cya at between 30 and 60 ppms - this way my chlorine is always between 3 and5 - I like to keep it around 5.
I like to use regular household bleach with a combination of trichlor pucks. This way I can regulate the ph (trichlor pucks are acidic) and the cya at the same time.
I use a sequestering agent, added at the opening of the pool season, and add as needed when new water is introduced through filling, because I fill with well water. This year I used Proteams Metal Magic and was very happy with the results.
At opening I add the sequstering agent when I open the pool before I put any other chemicals in. If I have to shock, I make sure the ph is no higher than 7.2. I keep my eye out for stains and add more sequesterer if I see stain forming. If the stains are too heavy, or are not lifted with sequstering agent and low ph, I do an ascorbic acid treatment. Once all of the stains are gone, I rebalance the water, and keep the chemistry in check. this ususally works well for keeping the pool stain free. It is very simple once you get to know your own pool, and you can do your own testing.
I will answer any questions anyone would have on stains - I hate stains
