Green algae at the bottom of the pool

  • Last Post 15 July 2021
Rafiolo posted this 12 July 2021


I had a DE filter lasts season but it broke and my landlord replaced it to sand filter . Since he did I'd every couple days I have like green algae dust at the bottom on my pool . Water is clean all chemistry is in correct level . I can brush the dust but in a day or 2 it comes back . I use algacide 60 but it doesn't help either . What else I can do ? Is it possible he messed up something while installing the filter ?

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JCMC70 posted this 13 July 2021


Is it possible he messed up something while installing the filter ? 

Anything is possible. But there's a way to check.

With the filter dial valve in the Filter setting, take the cover off of the skimmer:

1. Is water moving in the skimmer?

2. Is water coming out of the return jet(s)?
If the answer to those questions is yes, the filter is plummed correctly.

What else I can do ?  

You can provide more information.

Is this an above ground or in-ground pool.

If in-ground, is it a vinyl liner, a plaster/gunnite or a fiberglass pool?

If in-ground does it have a skimmer and a main drain?

How many gallons does it hold?

A complete set of chemical readings:

Free Chlorine 

Combined Chlorine or Total Chlorine 

Total Alkalinity 


Calcium Hardness

CYA (stabilizer)

Tell us if you took the readings yourself, (and if you did, did you use drop reagents or test strips) or if a pool store did the water testing.


DontTemptThePoolman posted this 15 July 2021

If your chemicals are all where they should be and the water isnt over stabilized or locked...

Dont underestimate brushing the bottom a few times during peak chlorination to allow the chemicals to reach all the matter.  Also stubborn areas inside the rims of skimmer or behind a ladder can have a build up of algae that is resistent and replenishes itself even amongst proper chemical levels.  Manual cleaning could be in order.  If this doesnt do it...

I have had an issue similar to this and never came to a certain conclusion as to the issue.  The leading theory was that there are some very fine particulates that seem to allude filtration.  I've considered adding a small amount of DE to the sand filter to see if this wouldnt clear up the issue but I havent tried it and I am not recommending this because I am not sure it would be safe for your system.  If I did do it I would keep a close eye on the system pressure and do some homework on how much to use.


JCMC70 posted this 15 July 2021

If the green dust is in fact an algae bloom, normal chemical (chlorine) levels won't get rid of it. Chlorine would need to be elevated to shock levels (determined by the amount of stabilizer in the water) and maintained until the algae is dead. Hence the request for complete chemical readings.

 The pool would need to be brushed and vacuumed daily to remove the algae. It can take days after the algae is dead to remove it all. Especially with a sand filter because sand filters only filter particles down to 20 microns.

Adding a small amount of DE to a sand filter will help with the filtering process and it doesn't harm the filter. DE filters particles down to the 4 micron range. However, if you add DE to the filter, you must check the filter pressure frequently because the filter pressure will rise quicker.

The correct way to add DE to the filter is to backwash the filter and note that filter pressure. Then, with the filter pump running, slowly pour 1/2 cup into the skimmer. Allow a minute or so for the DE to get to the sand bed in the filter and then check the filter pressure. If it has risen by 1 psi, you have added enough. If the pressure is still at the starting number, add another 1/2 cup and check it again. The amount needed will depend on the size of the filter but the rule of thumb is 1cup or less for smaller filters up to 2 cups for larger filters.

There are other products, like Fiber Clear, that enhance the filtering in sand filters. These products claim to filter particles down to 2-3 microns