Help - Black Algae - Shocking Pool

[tjf120]

I have black Algae, day 2 of super-chlor shocking. Quick history:

- Had pool water tested at store at first, thought they were doing an ok job. Became member on forum.
- Found some algae in pool, SWG had crapped out, green algae formed. Shocked pool, got rid of green algae
- decided to switch from salt to non-salt (chlorine, BBB method) due to erosion of rock feature waterfall on pool (separate post)
- drained half of pool and filled with tap water
- shocked pool based on pool store results (they were saying CYA = 100)
- Checked pool with FAS-DPD test kit, Results:
FC=25 ppm
pH= 7.5
Alk = 90 ppm
Hardness = 300 ppm
CYA= 50 ppm

So, I'm pretty happy with my chem results with the FAS test kit. However, still have black algae. I have scrubbed the spots with a steel wire brush, didn't see a change today.

Questions
- anyone had success with scrubbing spots with pumice stone?
- are my test results above accurate with that high of chlorine, ie, is the FAS test kit independent of FC levels?
- I have not backwashed my DE filter yet, should I?
- can I get in the pool with the chem results above and scrub with pumice stone? Or not recommended?
- I'm doing a FC overnight test tonight, will post results

 

[BoDarville]

tjf120:

are my test results above accurate with that high of chlorine, ie, is the FAS test kit independent of FC levels?

The FAS-DPD test will test FC at that level (25 ppm) and higher, so I would trust those results. However, your pH test is likely not accurate if you tested that while the FC was at 25. High FC levels will throw off the test results.

I have not backwashed my DE filter yet, should I?

If the PSI is 20% or more higher than your starting PSI right after your most recent backwashing then yes.

Haven't used pumice stone for anything, so I cannot comment on that. But someone will likely be along shortly who can

 

[tjf120]

I have not heard the 20% comment before.

IE, the pool after a backwash is ~10 psi. It is at about 12 psi (so 20% above original). On the 'dial' the number is ~20 psi, ie, ~10 psi higher than the original. You know on the filter, there are the 2 needles, separated by ~10 psi.

 

[duraleigh]

Black algae usually takes a LONG time to get rid of...perhaps weeks. Brush the spots as often as you can with a stainless steel brush and keep your chlorine elevated roughly twice what we suggest as normal. Eventually you will eradicate it.

 

[BoDarville]

I have not heard the 20% comment before.

Cleaning / backwashing the filter when the pressure is 20-25% higher than the starting pressure that occurs right after filter is cleaned/backwashed is a rule of thumb that many on the site, myself included, follow. The other visual cue that works for me is when I notice a slight reduction in the water flow from the spillover spa to the pool. That just happens to coincide with an increased pressure in the 20 - 25% range.

IE, the pool after a backwash is ~10 psi. It is at about 12 psi (so 20% above original). On the 'dial' the number is ~20 psi, ie, ~10 psi higher than the original. You know on the filter, there are the 2 needles, separated by ~10 psi.

I'm not sure I follow your setup here. On my filter, I have a single gauge containing one needle that is mounted on top of the filter. That is the reading I go by. I have recorded the starting pressure for when I have the valves set to recirculate water from both the spa and pool and for circulating within only the pool, since there is a difference in the pressure depending on how I have the valves set.

 

[tjf120]

Black algae usually takes a LONG time to get rid of...perhaps weeks. Brush the spots as often as you can with a stainless steel brush and keep your chlorine elevated roughly twice what we suggest as normal. Eventually you will eradicate it.

So with a 4-6 target of FC, keep it at 8-12? No issues with swimming in those levels?

If I do my borate additions on Friday, would that help at all (or just help protect against formation of new algae)