Stabiliser (isocyanuric acid) is a very misunderstood product. The purpose of stabiliser, formerly known as sunscreen is to hold chlorine in the water so it cannot be removed as easily by UV light.
There is extensive debate over the correct amount of stabiliser to use. The Health Dept. recommends between 30ppm-100ppm. In the US however this maximum is increased to 200ppm. In pools using normal unstabilised chlorine or with chlorinators a level of 80ppm-120ppm will ensure chlorine is not wasted. These maximums are arbitrary levels as no definitive maximum is known.
The only way to lose or lower your stabiliser level is to remove water.
Many people have heard of ‘chlorine lock’, a situation where there is too much stabiliser so your chlorine won’t work. This can occur in pools using stabilised chlorine (e.g. floating tablets, dichloroisocyanurate dihydrate or trichloroisocyanuric acid). When dichlor or trichlor are added to a pool they split to produce free chlorine and stabiliser. These can be useful when dosing a pool to remove algae or boosting a chlorinated pool with chlorine. If the level of stabiliser is too high when adding stabilised chlorine an incomplete reaction may occur not allowing enough free chlorine into the water. The simplest action to take is to dose your pool with more unstabilised chlorine. This will help break the reactions and free the chlorine into solution. This is why we do not recommend the continuous use of stabilised chlorine in swimming pools. It is cheaper to add stabiliser separately and use regular chlorine.
Most dichlor and trichlor products recommend on their packaging only to use them if the stabiliser level is below 50mg/L. If the level is above this you should use unstabilised chlorine. The only effective way to lower stabiliser levels is to remove water (dilution). If your level is a concern I recommend using only unstabilised chlorine products for the next 6 months and then reassessing the level.
If you are using excessive amounts of chlorine or the pool is getting green algae even with the correct chlorine dosage, especially during summer months, you should check the stabiliser level at your local poolshop. Test kits for stabiliser are available but it is not included in a standard test kit. 3kg of stabiliser at the beginning of summer each year should be sufficient for a 55,000L pool (standard size).