I'm struggling with persistent low-grade green algae, and either it's very resistant to chlorine or we have some well-hidden CYA (or something else) which is inhibiting the chlorine's oxidizing action at the levels recommended here. This is our fourth summer with our pool, and I've followed the advice of TFP (bleach as primary sanitizer, correlated with CYA levels) since we first started using it. The pool was a green swamp when we moved in, so on the first opening I drained, pressure-washed and refilled it. First year was great, no signs of algae for 3 months, then a big bloom which I eventually got under control (again following the advice of this site). Subsequent years I have blasted it with bleach on opening to kill off the algae but I seem to fight with algae growth continuously throughout the summer unless I SLAM it (and hold) to 2-3 times the recommended shock level.
Some pool facts:
- Pool is gunite/plaster (green tinted plaster, which makes it fun to see algae), about 17,000 gallons (just a guess, but its response to chemical additions correlates). Based on permit history, the pool is about 35 years old, and *may* have the original plaster (if that's possible). The plaster is in very bad condition--rough with lots of pop-offs, a number of rust spots, etc. Fortunately the pool still holds water quite well. We're planning to resurface after this season.
- Bleach: I use the 10% bleach from Home Depot. Seems to work okay, and it's readily available. 1 gallon raises the chlorine by 6 ppm.
- Cover: We use a floating solar cover. It's only 3 years old but is on its last season. The high chlorine levels from previous years significantly weakened it.
- Maintenance: I normally test the pool every 3 days, and add a half gallon of bleach. Doing it this way, the pool cycles between 6 and 9 ppm (higher than the 2 to 5 ppm recommended by the pool calculator) but I still get visible algae growth. I brush once a week. Tonight there were visible green clouds when I brushed. The pool is pretty clear and generally looks good, even when algae is starting to grow.
- Filter: DE filter, used exclusively with fiberclear (or equivalent) since first installed. All new grids at the start of the season. High chlorine levels destroyed the original grids in 3 years (broken embrittled plastic spines, holes in fabric).
- Pump: I run the (single speed) pump about 4 hours a day, which should be about 1 to 1.5 turnovers.
- Trees: The pool has a lot of oak trees (live oak, valley oak, black oak, etc.) around it, but I've trimmed them back so they're not directly over the pool. They don't drop a huge amount of stuff in the pool, and what they do drop tends to get picked up by "Squirt" (booster pump driven pool cleaner).
- Winter: I'm in California so I don't winterize per se, just raise the chlorine to about 10-15 ppm and then put the safety net and leaf cover over it. Try to run the pump a few hours a week, and add algaecide if the algae appears to be getting out of hand. Normally open the filter and clean the grids at opening.
- Borates: I added borates in the middle of last season (20 mule team + acid). It seems to work for its primary purpose of buffering rises in pH (but the buffering is asymmetric because it the pH seems to fall quickly), but it hasn't change anything with regards to the algae. If anything it seems to be worse this year.
- Combined chlorine: Always tests at 0, or at most 1 drop (so less than 0.5)
- CYA: For whatever reason, this always returns to 0 or close to it when the pool is first opened. It does not appear to be converting to ammonia, however, because the CC is always 0. I normally use some pucks of trichlor in a floater at the beginning of the season to bring up the CYA to about 30. I'd guess the level was about 10 at the beginning of this season, and I have added maybe about 3 pounds of trichlor so it should be around 20.
- Polyquat 60: I use this over the winter to keep the algae away. It works great, but it seems to load up my filter and make for short filter times between backwash until I remove and clean the grids. I considered using it tonight but it seems to interact strongly with the bleach--they destroy each other. Also, I don't want to break down the filter again right now.
- Phos-free: I've actually considered going to [BIG NATIONAL POOL STORE] and trying Phos-free just to try to lower the phosphate levels (I don't think they're particularly high) and help keep the algae under control. Please save me from this.
Tonight's test results (Taylor 2006): FC 7.0, CC 0, temp 82 °F, pH 7.3. Not tested tonight, but previously (and they're quite steady): CYA ~20, TA 110, CH 325, Borates 50
I dumped in 4 gallons of bleach tonight so theoretically the levels should be 31 ppm. I guess I'll buy more tomorrow and keep adding it to keep the levels up until it seems to be gone.
- Why don't the recommended chlorine levels from the pool calculator keep the algae away, even after SLAMing it to 3x the shock levels? I don't want to keep destroying my equipment with high levels.
- Could the rough plaster surface be causing much of this problem? (I can see algae hiding in the edges of the pop-offs). If so, we're considering a pebble surface for the resurfacing. Wouldn't that rough surface have the same problem?