Output Keeps Dropping Quickly; Lots of Algae

Mark Shvitz

Active member
Jun 20, 2018
Gainesville, FL
I'm new here. Hope this is the right area for this post.

I have an in-ground pool with what appears to be a big Sta-Rite sand filter. I am having some strange problems.

I got a little algae a while back, so I shocked the pool and brushed it. It cleared up and came back, so I shocked and brushed the pool again. Both times, I added some sort of blue algaecide to the pool.

Things have gotten worse, not better. The pool is behaving strangely. I have a waterfall, so I can tell how well the pump is working by looking at the output. The output is low now, as if the pump is clogged up. I backwash the filter, and the waterfall output becomes normal for a short time. Less than an hour. Then it slows down again. Backwashing again fixes it for another hour or so. I can't tell you anything about the pressure, because until today, I couldn't read it. I got a can of Off and cleaned up the pressure gauge lens, so I'll know more tomorrow. I am pretty sure the pressure is rising after the pump runs a while.

It's as if some kind of crud is in the water, clogging the filter almost as soon as I turn it on. I'm wondering if the algaecide is to blame. I never had this problem until I used it.

Any idea what's going on? The pool is getting very nasty, and I can't keep backwashing it 5 times a day.

I am not a great pool boy. I moved to this house last year. I did not want a pool, but the only property that worked for me had one, so I'm stuck with it. Frankly, I do not like pools and wish this one were a nice grassy area with a barbecue pit. I don't care if the pool is fit to swim in. I don't care if the chlorine burns people's eyes. I just want it to look good and be trouble-free.


Gold Supporter
TFP Guide
Jul 21, 2013
Northern NJ
It will take some time and effort to get your pool trouble free. Your filter is doing its job in capturing the algae. But if you don’t get ahead of the algae growth the cycle will never end.

You need a good test kit such as TF-100 Test Kit â„¢

Then you need to do Pool School - SLAM - Shock Level And Maintain to kill the algae. You can put your filter on recirculate while killing it with CL so you dont need to clean it every few hours. But eventually you will need to filter.

Your pool is not clear until you pass Pool School - Perform the Overnight FC Loss Test (OCLT)

Once you get there we will work on your normal pool care.


Bronze Supporter
Apr 27, 2018
Midland, MI
One of the first thing you need to do as suggested by Allen is get your own test kit. We recommend the TF100 from TFTestKits.net or the Taylor K-2006C.Once you get your own test kit, please post your numbers here. You should be posting results for:

FC - Free Chlorine
CC - Combined Chlorine
PH - Acidity/Basicity
TA - Total Alkalinity
CH - Calcium Hardness
CYA - Cyanuric Acid
ABC’s of Pool Water Chemistry

Your algae keeps coming back because you're not maintaining your FC shock level. Please read up on SLAM. You have to maintain your FC shock level which is dependent upon your CYA results of your pool. You also should have a pH balance of 7.2 before SLAMing your pool. To help you further, you should get the test kit in a timely fashion and post your results here.

In the mean time, as you wait for your test kit, please clear you pool of debris using pool tools such as a brush, vacuum, pool net , etc. If your pool is really dirty, it is recommended that you vacuum to waste to avoid clogging up your filter. Scoop out as much junk on the surface as well as on the bottom. By doing this, you’re allowing your chlorine do to its job on the algae. If your pool is green now, add about a half a gallon of 10% liquid chlorine until you get your kit. That will add close to 5 ppm FC per day.Here are some articles for you:

Pool School - How to Chlorinate Your Pool
Pool School - Chlorine / CYA Chart
Pool School - Recommended Pool Chemicals


Well-known member
Jul 4, 2011
Perrysburg, OH
I don't care if the pool is fit to swim in. I don't care if the chlorine burns people's eyes. I just want it to look good and be trouble-free.
Honestly I'm not sure if this is the right method for you. This method can be trouble free but will take some time and effort getting there. We are all committed to ensuring our pools are extremely fit to swim in and nothing burns peoples eyes.

Mark Shvitz

Active member
Jun 20, 2018
Gainesville, FL
If I could snap my fingers and make the pool disappear, I would do it. I can't, so I want as little aggravation as possible. One of the problems with buying a home in Florida is that you're likely to get stuck with a pool whether you want one or not.

Mark Shvitz

Active member
Jun 20, 2018
Gainesville, FL
My pool is free of algae. I have been running the pump around the clock. I have been backwashing it once or twice a day. The flow still drops within a couple of hours of backwashing. I can't figure this out. What could be clogging a filter over and over, within a short time? Very little gets into the pool because it's screened.

I have been vacuuming. There was a lot of grayish silt on the bottom after the shock worked. My problem now is that I can only vacuum for a short time before I have to stop and backwash to get the flow going again.

Mark Shvitz

Active member
Jun 20, 2018
Gainesville, FL
I found out one of my filter elements has a small tear in it, and the bleed apparatus had fallen off due to age. This was what caused crud to appear on the bottom of the pool and gave the algae a new place to grow. DE was leaving the filter and shooting out into the pool. I have ordered a new filter element and a new bleed tube. Hopefully this ordeal will end when the parts arrive on Wednesday. Unfortunately, I probably shot a good deal of brand-new DE into the pool today, before I found out what could happen with a torn element and missing bleed tube.