Michelle W,
Googling "Chlorothalonil Algae" it appears that this product can create algae blooms, which is the main ingredient in "Bravo Weatherstik".
In most cases this was as "Wikipedia" states:
"The loss of these herbivores and plants freed the algae from predation and competition, which eventually resulted in algal blooms that were similar to the effects of eutrophication."
But that reasoning is unlikely in your case, as you had virtually no living organisms to kill in your pool.
From looking at other papers it does not seem to affect the Phosphate levels to any degree of importance. So you could rule out Phosphates, and even at levels of 3,500 ppb sufficient Free Chlorine levels will control any effects it might have on algae.
However, if you go through the Google list you will note that there have been reports of elevated nitrate levels as mentioned by "Brushpup", this mighty be the reason for a light algae bloom, with this product. You can test for this with an inexpensive aquarium nitrate test kit. These nitrate levels, according to research papers, seem to build up slowly but surely over a period of time.
Nitrates can be controlled in a number of ways the two easiest ways are, by draining but maybe you do not wish to do that, or secondly by raising your chlorine level to the high side of the maintenance dose (but below shock levels) in combination with a slightly lower pH of say 7.6 or 7.4, this makes the chlorine more effective.
With a CyA level of 40 ppm, a level of 7.0 ppm of Free Chlorine, should do the trick, but no higher than 15.0 ppm, as shock level is 16, for daily maintenance. To clear the present bloom you will need to shock the pool.
I am not stating it is Nitrates for sure, you will need to test for them. Higher levels of chlorine should do the trick, if they do not I would ask the local agricultural office to test a sample of your water to see what is in it.
Finally you reported your FC level as 8.5 and your TC level as 5, that as stated by "Isaac-1" cannot be, "Isaac-1" stated " The math does not add up there, TC is total Chlorine and equals FC+CC".
I am guessing that your TC was 8.5, and your FC (Free Chlorine) was 5, please double check your results before you do a shock process, as my CC calculation could be a wrong guess This would give you a Combined Chlorine of 3.5, using this formula "TC - FC = CC". As stated in this site's basic pool chemistry section (bold done by me):
http://www.poolcalculator.com/chemistry.html#FC
CC - Combined Chlorine
Combined chlorine is an intermediate breakdown product that is created in the process of sanitizing the pool. CC causes the "chlorine" smell many people associate with chlorine pools. If CC is above 0.5 you should shock your pool. CC indicates that there is something in the water that the FC is in the process of breaking down. In an outdoor pool, CC will normally stay at or near zero as long as you maintain an appropriate FC level and the pool gets some direct sunlight.
Your CC level,
if it is 3.5, is too high, and therefore you will need to shock, preferably with liquid bleach, and sufficient amounts of muriatic acid to compensate for the pH rise created by the bleach.
Each 1 ppm addition of liquid bleach requires 1.8 oz of muriatic acid to compensate for pH rise, based on 10,000 gals
Here are Taylor's instructions for doing the test with their R-0001, R-0002, and R-0003 chemicals:
Chlorine (Free, Combined, Total) Test
1. Rinse and fill small comparator tube to 9 mL mark with water to be tested.
2. Add 5 drops R-0001 and 5 drops R-0002. Cap and invert to mix.
3. Match color with color standard.* Record as parts per million (ppm) free chlorine (FC).
4. Add 5 drops R-0003. Cap and invert to mix.
5. Match color immediately. Record as ppm total chlorine (TC).
6. Subtract FC from TC. Record as ppm combined chlorine (CC). Formula: TC - FC = CC.
Most importantly I hope you are doing the chemical testing yourself with a good kit such as the one sold by this site or a Taylor's kit, just make sure the chemicals are not out of date by calling either supplier. Pool store tests can be very unreliable.
Good luck
