Algae Spots form in SLAM

Van G

Bronze Supporter
Mar 22, 2016
Long story short - I had algae show up in spots when my FC dipped down to 1 after a storm. I slammed earlier in the season but in hindsight, I didn’t clean the cartridge filter since my pressure is always good (rookie!).

So this time I opened up the cartridge and sprayed them to near new white, vac’d out all water/sludge, and cleaned all parts & surfaces. It was pretty bad! I also pulled a mouse out of the skimmer basket...

I’m day 2 of my slam and things are going well in terms of FC consumption and water appearance. I’m slamming at 20 (pre-slam CYA 50, PH 7.5, TA 70, Salt 1800, Temp 70).

I’m guessing I never really got rid of the algae last time since my cartridges had a good amount of green on them. Just kept it at bay with target FC of 4?

My concern is this morning I had a couple spots of algae in different areas of the pool. That was with FC starting at 20 last night and dipping to 12 this morning.

I’m brushing a min of once a day and dropping robot vac in for 2 cycles a day.

FC only dropped .5 over 3 hrs from my last log/addition before bed so that is encouraging.

Does the Algae this morning make sense?
Could I possibly see it again with my FC levels?

Was the cartridge filter cleaning adequate or should I have done more? TSP soak?

Anything else I should do than keep going to pass OLT (maintain FC, brush, vac)?
Last edited:


Mod Squad
LifeTime Supporter
Jan 17, 2012
Evans, Georgia
As long as you're seeing new algae keep the SLAM process going. I wouldn't bother to do an OCLT until the algae all appears dead and gone via filtering and the water is clear. Remember that algae are resistant little buggers and they can hide under biofilm pretty good. Hence the brushing to disrupt that, even if you aren't seeing algae where you're brushing.
Make sure to check your dates on the bottles of liquid chlorine.....old or improperly stored liquid chlorine is weak chlorine.

Maddie :flower:


Gold Supporter
LifeTime Supporter
Jul 16, 2010
Portland, Oregon
Is your pH really 5.5?

If so, you're swimming in water that is somewhere between black coffee and urine acidic. Although not quite tomato juice.

Oh, and clearly algae can grow at your FC levels since you're seeing it. SLAM is a process, not a one time application that will magically kill all the algae. You need to maintain SLAM level FC for it to work. Whenever you lose FC under slam, you're risking letting it grow which is why TFP says to test and add chlorine 3 or more times a day to maintain SLAM level FC.
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Gold Supporter
LifeTime Supporter
Jul 16, 2010
Portland, Oregon
K... so you SLAMmed it, but didn't maintain.

What you saw is chlorine eating algae. Thus the major FC loss. Don't question it, just maintain SLAM level. That's the chlorine doing it's job.

Maintain SLAM level.

3 or more times per day, test and add back to SLAM level.

All your other questions will be answered if you perform SLAM as TFP teaches. It will solve itself soon.

Don't bother to get up and perform an OCLT till you pass the first two criteria. Clear water (no algae, clear like you can tell the side of a nickel in the deep end), <.5ppm CC, and pass an OCLT.