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Thread: Black Algae Behind Pool LIght

  1. Back To Top    #1

    Join Date
    Jul 2015
    Okanagan Falls, BC, Canada

    Black Algae Behind Pool LIght

    I made a big discovery today, behind my pool light there was a very large deposit of black algae. Since last year I have been chasing a chlorine demand/ loss of CYA issue brought on by the premature failure of a new salt cell last year. The pool generally looks very good, clear water, no stains (as long as the SWG is working). This year I did a lot of research into disappearing CYA levels and came to the conclusion that it was caused by an algae type issue. I recently completed a 2 1/2 week SLAM process and the pool was in good shape.

    During the SLAM I found some black algae around the pool light and brushed it clean. At this point I will tell you I thought the pool light was dry in behind bulb, now I know that is not the cause. I would not have found this problem if my Aqua Rite controller hadn't quit working early this week (burnt out current limiter, new one on the way from Digi-key). I started adding bleach this week and noticed I was consuming lots of chlorine again. I checked the light and gave it a tap and the black algae started coming out from the light. This prompted me to investigate further.

    I have cleaned the light and the niche good. I vacuumed what I could as I took it apart and the robot is running presently. After the robot is done I will manually vacuum what the robot missed and then take the filter apart and clean the cartridge filters. I have brought the chlorine up to 16.5 ppm, reading the bit on black algae in Pool School this should be a good level to hold for the next few days. Below are my test results from this morning. The PH is high I will lower with muriatic acid after I clean the filters.

    One thing I am finding is on my pool light there is a SS plate on the back of the light and there seems to be a build up of algae on areas I can't reach. I am soaking the light in a bucket of pool water with some extra bleach hopefully this will clear it up.

    The funny thing is after I found the problem and start search through the forum I came across many post with the same problem. I am sitting here type and shaking my head because now it seems so obvious what was causing my issues. I thought I had a good handle of dealing with pool issues before I am discovering it is a evergreen process in managing the pool.

    Please feel free to post any advice or comments. I have a bunch to get done as it is family pool party day as it is going to be 35c here today.

    FC: 16.5
    CC: 0.5
    PH: 8.0
    TA: 220
    CH: 325
    CYA: 40

    Cheers Ted

    55k L (14,500 gal) Fiberglass Pool, SWG, 430 sq ft cartridge filter (4), 1 1/2 hp pump, iRobot Mirra 530 cleaner

  2. Back To Top    #2
    Mod Squad Texas Splash's Avatar
    Join Date
    Jun 2014
    South-Central Texas, Marion/San Antonio

    Re: Black Algae Behind Pool LIght

    Ted, you did well to find that algae. Many folks refuse to check behind there and later discover the mess that was hiding there all along. I think you're on the right track.. The only thing I was going to say was in regards to your PH test. No need to get too excited about the PH reading since your high chlorine (FC) readings will skew that PH reading. Probably best to just continue with your SLAM then adjust the PH afterwards. Good luck, and hope your party went well today.
    Pat (a.k.a. Texas Splash) ~ My Pool: Viking Fiberglass; 17,888 Gal; Waterway Supreme 2-sp/2-hp pump; Hayward Ctg filter; TF-100 w/ Speed Stir
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