just shocked pool, now what?

[SW FL]

i was suffering a bad outbreak of mustard algae. after reading several posts & replies I decided that a shock was in order. did a 5lb shock treatment followed by brushing & 24hr pump. water is now free of algae. i've washed the filter & re brushed. check chemicals every 6hrs while this was going on and want to get the chemical balance to normal but the chlorine test makes the sample go red. is this too much chlorine or not enough? i'm assuming the shock was using chlorine so i expect there would be a low chlorine level.

have 2 1/2g of liquid chlorine ready to go. so suggest on.

[chem geek]

The shock level to get rid of yellow/mustard algae quickly is a Free Chlorine (FC) level that is 60% of the Cyanuric Acid (CYA) level and in this case one should lower the pH first, to around 7.2, before adding a hypochlorite source of chlorine (such as chlorinating liquid, unscented bleach, or Cal-Hypo). If you have a high CYA level, then such high FC levels can be impractical so a partial drain/refill to dilute the CYA is usually required or other approaches using algaecides (e.g. sodium bromide) are needed.

If you truly got rid of the yellow/mustard algae completely and if it isn't getting reintroduced into the pool, then maintaining the standard minimum FC level of 7.5% (normal target around 10% or so) of the CYA should keep regular green algae away. If, on the other hand, yellow/mustard algae is still present, then it takes a minimum FC level of around 15% of the CYA level to keep it from growing. These numbers work for phosphate levels up to around 3000 ppb (which is high). If phosphate levels are much higher than that, then use of a phosphate remover (which is expensive) might be needed.

Other alternatives to keep away algae, other than maintaining sufficient chlorine levels relative to CYA, are to use a regular weekly algaecide such as PolyQuat 60 or to use a phosphate remover (large dose first time, then maintenance dose after that) or a copper-based algaecide (but this can stain or cause green hair in blonds at higher pH).

Richard