My suggestion would be to add borates to 50 ppm. 'Borates are an algaestat and I know firsthand that they work. I live in Florida and go away for about 10 dayd to two weeks every July and I just shut my pool completely down while gone (not worried about it being green when I get back because it's easy for me to clear it.) Since I added borates about 4 years ago my pool is not green or even cloudy when I get back. All I do is shock it before I leave. There is no chlorine when I return but I just shock it again and I'm back in business. IMHO, it is a more cost effective solution than weekly additions of polyquat 60 and it has the benefit of improving the 'feel' of the water and helping stabilize the pH because of the secondary borate buffer system.
Here is background on my borate experiment:

http://www.poolforum.com/pf2/showthread.php?t=5419

http://www.poolforum.com/pf2/showthread.php?t=4712

Here are instructions on using 20 mule team to add borates:

60 oz. by weight of 20 mule team borax will raise 1000 gallons of water 50 ppm and requires 30 ounces of 20 baum (31.45%) Muriatic acid to maintain the pH.
(12 oz. borax raises 1000 gallons 10 ppm and needs 6 oz acid)

Half the needed amount of borax is dumped into the pool and then half the required acid is then added slowly in the return stream (or diluted in a 5 gal bucket of pool water one or two quarts at a time and then poured into the pool).
Brush the pool to mix the chemicals
Add the second half of the borax and the rest of the acid, brush again.
Circulate for 24 hours non stop then return to your normal pump run time.
Check pH and borate level in 48 hours. If pH is low but in range you are done, If it is high, lower it to 7.6
Actually, I have checked my pH and borates about 2 hours of continuous circulation and if the pH is anywhere between 7.2 and 7.8 I have gone swimming.

For testing borates the LaMotte borate test strip have proven themselves to be superior to all the other strips on the market and provide enough precision to get your borates into the recommended window of 30-50 ppm. I have checked them against the Proteam borate titration test kit time and again they are certainly well in the ballpark! The Proteam test kit is expensive, difficult to find, and not the easiest test to do, btw!
The AquaChek borate strips are much more difficult to read than the LaMotte and seem to have a much shorter shelf life.