So, 18.5 ppm CYA per month. That's about 120 ppm in a 6-month season so starting at say 20 you end up with 140 if there's no dilution. That's high, but in a vinyl pool you aren't risking plaster degradation that can happen with CYA above 100. At least you clear out the CYA each year over the winter (by letting the FC go to 0). If you don't maintain the higher required FC level, then definitely use an algaecide and PolyQuat 60 is the best. It's not cheap, but it should work to keep away algae.

If you are using pucks because you aren't around (or don't want to) add chlorinating liquid or bleach every day (or don't have a pool cover so you can add it twice a week), then you might consider using Cal-Hypo slow-dissolving tablets instead (not in the same inline feeder as Trichlor -- it's an explosive mixture -- but in a separate feeder, perhaps a floater). Building up CH from the Cal-Hypo isn't as serious and can be diluted once a year.

Richard