I let my pool get green over winter. Bought some magic Algaecide potion from the pool store, additionally applied some 1-2 lbs of powder shock at the time and within a day or two all green was gone and had turned into gray matter. Cleaned filter, backwashed a few times and all was good. That was back at the end of February.
Ever since then, I've been maintaining and monitoring the chemicals in my pool quite diligently. Never let FC go below 7.5% of CYA.
And yet, a couple of days ago, I got a faint film of algae in the deep end. How's that possible with a steady 0-0.5 ppm CC over the past month+ and FC within tolerances relative to CYA?
CYA level is quite volatile due to a leak, that I estimate somewhere between 500-1000 gal of water per week. I have an autofill device that keeps my pool water level topped off at all times. So, CYA is a consumable for me and I add about 2 lbs of it every 10-14 days. I use liquid chlorine (10.5%) for sanitizing. CH, TA, and pH keep in check all by themselves, so I don't have to do anything about them. I only add CYA and liq. chlorine.
Full disclosure: I don't scrub my pool walls as often as I should, maybe once a month. Could that be a reason for the green film?
My last pool test results were:
FC: 5.25 ppm
CC: 0.5 ppm
CH: 200 ppm
TA: 120 ppm
CYA: 40 ppm
Water: 24°C (=75°F)
[Oh yeah, in case you're wondering: the 5.25 ppm FC test result is correct. I double my water sample size to 20 ml for the FC/CC test, so each reagent drop counts as 0.25 ppm.]
My test results over the past weeks since reopening my pool in February are available as an interactive graph (courtesy of Google Sheets). Note that two y-scales are used and some of the values have to be multiplied by 10 (water °C and CYA ppm). Only CA and TA use the right y-scale.
The saber tooth curves of FC at the end of Feb/beginning of March reflect an additional test three hours after adding chlorine to give me an idea of chlorine usage per day. Only one more test in March (on the 21st) was like that. All other FC readings were taken at around 6 p.m., the lowest reading for that day, immediately before adding chlorine.
I always add 0.5 gal chlorine per day (these appear as faint green bars in the graph), but skip a day if the FC result is high enough. Stabilizer added appears as a red bar, and so far I've been adding it in 2 lbs doses.
The recent rapid drop in CYA from 70 ppm to ~45 ppm within just two days at the end of March has me puzzled. Could that have been from the rain?