CYA (cyanuric acid) and pool maintenance

Having read some of the posts here I am curious to know more about CYA (in plain English please). Do I need to buy anything to deal with it? I read about test strips (not a good idea) and a testing machine (which one), but am really none the wiser.

I have only had my pool since last August and we ran it successfully last year. We are contemplating clearing the pool of all the green algae at the end of May ready for the summer, but appreciate that we may need to have a second clear up when we return at the end of June.

I have read about putting chlorine in to kill the algae and then hoovering up the dead stuff. We got no help from the company we bought the pool through and don't want to go back to them following a dispute.

Any help would be gratefully received as instructions were not their (the company's) strong point and we are very much novices!

Well done! Sorting the pH is essential - needless to say I didn't even think of mentioning it because it is a given (sorry!) my water here is extremely alkaline & takes a good few days of adding pH- to come down to the value where chlorine etc are effective.


Thanks so much for your advice. We have now sorted out the pool - at least we think we have. My husband is a Chemist (not the pharmacy type!) and he picked up on the PH as being important in relation to the chlorine level.

I have posted our “battle” details below with photos of the “clean” pool one week after the start of the work. My husband swam in the pool yesterday and said that it was good, although very cold when he got out!!

Thanks to everyone’s advice we will hopefully be more prepared for it should it happen again!

Hi Veronique. Thankfully we have cleared our pool without having to empty it. It has been hard work, but it seems to be pretty much back to normal.

Have a look at the photos I have posted.

It has been a tough battle, but we are almost there.

We skimmed off the brown gunk, added liquid PH- to get that level right, added liquid chlorine (3 x 10 litres), added algicide followed by floc and then swept the bottom twice.

The pool is not perfect yet, but both the chlorine and PH levels are spot on.

It has taken 6 days and we are having to add a reasonable amount of water, but it looks almost as good as it did when it was new last year.

We have bought some large tarpaulins, to cover the pool while we are away, and we have also bought a floating shock for while we are away for a couple of weeks and the power is off.

Fingers crossed it looks as good when we get back.

The pool Industry sells anything and the mesh is actually industrial geotextiles fabric the sort used on motorway banks etc.
Don’t bother with algaecide just not strong enough and chlorine is quicker.go and buy 4 20 litre bidons of eau due navel from a brico. Add 10 litres of navel asap then add 5 litres 12 hours later and again 12 hours after that. Keep the filtration on constant for now. Brush the walls and floor and be patient.
Let the battle commence

Hi John,
Our winter cover is a green mesh, so not really light excluding. We were only away got just over 4 weeks and threw in a lot of chlorine to hopefully keep any algae down.

Could we use liquid algicide and then use the floc? We have got rid of the brown crud, but the water is still green. We can see the top two steps. We put more chlorine in today as the stuff we put in the other day has been used up.

We really hope not to have to empty the pool as we only filled it 9 months ago!

Oh dear, I would empty and refill as well. Quickest way to a usable pool. Heavy, light obscuring covers AKA winter covers are not just for winter, they should be in use whenever you will be and away for more than a week. Slow release galets can be used when you will be away for up to a week. Adding chlorine, an extended period of absence and no cover gives exactly the stagnant pond you have now.

Don’t use peroxide, it is a stronger oxidiser than chlorine but a weak bacteriacide. So you burn out the chlorine and can’t get chlorine levels until the peroxide is done.

The problem with magic is its all illusion.

Good grief. Were my pool to be like that I'm afraid I'd empty, clean, scrub etc and refill ie start again from scratch (I have had to in the past). Sorry not to be more help. I usually use eau de javel for killing algae but have sometimes resorted to hydrogen peroxide which is magic stuff.

Hello John,
You didn’t get back to me about which testers for CYA might be best. Would it be possible for you to let me know?

At Easter our pool looked promising - just a little algae, but the water was lovely and clear. We threw in some chlorine to tide it over until we came back.four and a half weeks later we are back to a sludges green/brown pool. We were devastated!

We have thrown in some more chlorine as the levels were probably at zero. We have cleaned the cover and disinfected it ready to dry it out and put away fro the summer. Martyn has begun to clean the sides down and we have topped up the pool so that we can put on the skimmer and the rest of the system (UV filters).

Are we doing the right thing?

What would you recommend we do as we are going back home in just over a week and a half, but only for two and a half weeks. We really don’t want to leave the filter system running as we have had issues with our power blowing and rotten food etc.

Any suggestions?

I will shortly post an overview on some of the currently available chemical testers, from the electronic ones to the colour matching ones and the merits and pricing. Buying off Ebay may seem cheap but they won't help with advice and if buying in from the USA by the time shipping and import duty have been paid the price is very similar but EU instruments have a warranty US units don''t. I don't propose an exhaustive list as I don't intend to get involved with units that do not meet basic requirements of cost and features. If you want a high end machine email me for more info.

With your fresh start up last year you are unlikely to have any CYA in your pool unless it was dosed in by Thermapool which I doubt. Each chlorine tablet (Multiaction Galet ) adds chlorine and some CYA, you need this to prevent the sun's UV from burning off the chlorine too quickly. It could take nearly a season to add the CYA this way and you will be losing chlorine all the time. The half life of chlorine in a pool without CYA is around 35mins so the chlorine is all gone after 2-3 hours that means algae can grow as you have found out now. To rid the pool of this algae you need more chlorine.

You must keep a residual level of chlorine in the pool at all times unless a winter (dark, light obscuring cover) is in place.

After a while using galets the CYA figure will reach 30-50 ppm (parts per million) at that point you have about the maximum protect CYA can give you but the residual level of chlorine need to also be increased slightly as most of the current chlorine will be chemically bound to the CYA so increasing the free available chlorine level to 1.5-2.5ppm is what is then needed to provide a residual to counter algae and CYA and keep the pool clear. Chlorine is still burnt up by the sun's UV and also after rain showers etc so must be replenished, this can be done in an erosion feeder (flying saucer shaped item in which you put chlorine tablets) this will allow the tablets to dissolve at a steady rate.

If you have a salt water chlorinated pool you still need CYA so can use the galets or a liquid or powder version of CYA dissolved separately into the pool but it will be needed or the cell plates producing the chlorine will be working harder to produce chlorine more frequently and they will wear out quicker and the cells are not cheap.

To shock a pool full of algae you don't need products called shock (Choc) you need to raise the chlorine level to a point where all the algae is killed off and that can take a fair bit of chlorine and some patience, as the chlorine will get used up quickly killing algae so it needs regular top ups to maintain the high level until the algae is dead and oxides/filtered out of the pool and the chlorine level remains in the pool steadily without dropping. We use the combined chlorine and over night chlorine tests to determine when shocking is complete.

It's not a spectator sport either, you need to run the pump and brush the side and bottom frequently and backwash the filters (rinse out the filters or bags if you have them).

The only plus point of no CYA is the chlorine is a bit more powerful without CYA so shocking a pool is easier but because your chlorine can get burned up quickly shocking in the evening when the sun isn't as strong is a good idea but if you have algae, do not wait get the chlorine in there asap but add more ad sun down.

A good cheap source of liquid chlorine is Eau de Javel in 20ltr biddons from a Brico shed, Brico Depot usually stock this for €12.90. Being a liquid it doesn't waste time dissolving and contains no CYA, this means it's great for dosing your pool all the time.

Hi Freya

Welcome to the SFR discussions on swimming pool maintenance. There are loads of previous posts on the subject of swimming pools, and I have hugely profited from the wise words of John Withall, who I'm sure will pick up on your query.

Assuming you are sanitising with chlorine based products.../

CYA (cyanuric acid) is a stabiliser included in commercial pool sterilising products such as Chlor Choc and Chlor Lent. Basically CYA slows down the evaporation of the chlorine resulting in the chlorine to do it's job over a much longer period.

Problems arise with algae when over a period of time the CYA builds up and in effect over stabilises or blocks the chlorine. And, that's where test strips are useless, as they can indicate high levels of chlorine whist at the same time the pool water is gradually turning green! I learnt to my cost, and had to do a complete water change last year!!

You need a decent testing kit such as the ColorQ (check it out on e-bay) which will indicate the levels of chlorine and CYA, and also advise on the correct levels required. You also need to maintain the correct level of PH via PH- or PH+, as it facilitates chlor to do it's job.

If you find that the CYA level is too high, you can replace a quantity of the water until the CYA level reduces, or/and add some free chlorine (no stabiliser) such as 'domestic' javel or calcium hypochorite to correct the CYA level and halt the march of algae.

I hope that helps, but am certain that pool guru John will impart his own and greater knowledge and advice.

Good luck