Can't get rid of Black Algae


New member
Sep 25, 2018
Ramat Hasharon, Israel

My pool is in ground, 19,000 Gal, SWG, Mosaic covered.
My algae problem started a year ago when I was on vacation for a month and my pool man probably didn't appear to clean the pool - when I got back from vacation it was all black.

To solve this, he dropped 10Kg / 22 lbs HTH to the water and once they become clear (HTH makes the water all white and cloudy), the pool became clear of Algae.
Two week later Black Algae strated growing again, and since my poolman suggested the HTH treatment again, I decided to start learning how to maintain my pool myself, mostly based on this wonderful site.

So, from last September I'm the poolman. Needless to say that once I started testing water chimestry, I found out the CYA was 0, FC was 0, Salt level was 1.8 so it was really a mess. I managed to level everything per the suggestions on your web site (also using the Pool Math app).

Winter went alright (I'm in Israel, our winters aren't too cold) - water was clear and I maintained CYA at 60-70, FC at 3-5 and PH 7.5-8, TA 80-100.

My problems started as soon as water temperature went up to around 75F and Black algae starter appearing again.
I Slamed the pool based on your recommendations, maintained FC at 24 in the mornings (it dropped due to sun / Algae to about 16 in the evening), tested for FC every evening and morning (before the sun hit the pool) until FC loss Evening vs Morning was 1ppm (or less).

After Slamming, I decided to raise my idle FC level in the pool to 7-10ppm. The pool was free from Algae for about 3 weeks but Black Algae started reappearing last week.
I Completed another SLAM session two days ago and this time decided to keep FC level at 13-16 to see what happens but this morning, just two days after the SLAM, I see new Black Algae started building again.

Each slamming session involves the pump running 24hrs, cycling water through the filter and twice a day brushing the pool with metal brush.
Since we do not have Tylor in Israel, I'm testing FC levels with lab equipment from Lovibond which should be pretty percise.

Although I'll be happy to get expert ideas about what it is that I'm doing wrong, I have some thoughts / questions I'd like to ask the community:

1. I run my pool pump during the evening + night, each day for 14 hours as this is the time my SWG unit needs to produce enough Chlorine to compensate on the day time loss. My pool has a motorized hard cover. Does it make sense to cover the pool during the day so chlorine level loss by the Sun will be reduced and I'll be able to reduce the pump running time? My calculation shows that the pump needs 4 hours to cycle all the water, so if I can get it to work 6-8 hours, that will be a lot of every saving.

2. What's the effect, if any, of closed pool on Algae? In other words, does the Sun help us sanitize the water, or is it just consuming chlorine? if the pool is darkened by the cover, will it help maintain the pool free of Algae, given that water chimestry is as it should be?

Thank you,



Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
In The Industry
Apr 1, 2007
Sebring, Florida
Welcome to the forum:wave:

1. Yes. Cover your pool

2. Yes. Covering your pool will reduce chlorine loss tremendously. Please don't do it all day everyday but rather give your pool a chance to "breath". Probably around 10 hours weekly uncovered would be enough. The UV rays from the sun help reduce CC's so you need SOME sunlight.

Your description does not describe black algae........sounds like plain old algae to me. Tell us why you believe it to be black algae?