Hello all. I live in south Texas. I have an in ground 9000 gallon plaster pool. The pool was re-surfaced three years ago so the water is not too old. It has a Hayward cartridge filter with an in line chlorinator. I have had this house and pool for five years and NEVER had a water problem. This was a very heavy spring for oak and there are several oak trees surrounding the pool. It was a leaf trap this spring but other than that there have been no problems. About a week and a half ago I started to notice some light greenish algea on the sides and steps. It would brush away easy enough but comes back the next day. Not heavy, just a light coating . Shock would normally end this problem. I began to shock the pool with little to no results. I have a Taylor test kit and tested the water often. I would normally see a drastic chlorine raise after shocking but this time the test results still showed a very low chlorine content. I continued to shock, brush, clean the filter and fight this problem. I can’t get the chlorine level to rise up to levels high enough to kill off the algea. Four days ago, fearing a CYA problem, I took a water sample to Leslie's Pools to have it tested. Here are the test results:

CYA: 40
FAC: O
TC:0
TA: 110
PH: 7.6

Total dissolved solids were 250
and phosphates were 0

I was told my chlorine was probably unstable and given three one pound bags of "Free and Clear" which is an oxidizer according to the package. I was told to treat the pool with the oxidizer which should free up the chlorine. Last night I still had slight algea and tested the water and found it had a very low (1.2) chlorine level. I know chlorine is needed to kill the algea. I have continued to treat the pool with about two pounds of shock and a gallon of bleach each time I shock over the past several days and can’t get the chlorine level up to normal shock levels. At this rate I am afraid that I might need to drain it but am afraid of what the real problem is. The filter is green each time it is washed out from trapping the algea after brushing so that is working. Returns are all flowing at what feels like the normal rate and pump pressure is normal. I am at a loss to figure this out. Any advice?