Superchlorinating A Swimming Pool

by Jane
(New Zealand)

If my pool is 80,000 litres,what is the calculation to superchlorinate with calcium hypochlorite (HTC)?

Is it better to use sodium hypochlorite and how long after I've shock dosed the pool can I re-open it for?


Thanks for the question Jane

We're still on standard measures so I'll need to convert your pool size.

80,000 litres is about 22,000 gallons of water. It takes 2 1/4 gallons or 9 litres of chlorine to shock your pool with liquid chlorine.

For granular chlorine, or calcium hypo. it would take 1.6 kg to do the job. If you're going to shock the pool I'd recommend using liquid chlorine. Calcium hypo does a good job but it will leave a white film of calcium on the bottom of the pool that will need to be vacuumed up later.

Dilute the chlorine in a large bucket with pool water then broadcast it around the pool. Be sure to sweep very well and keep filtering.

As for how long it takes to swim, that's a matter that depends on the sun and heat. You'll want to swim only after the chlorine gets down to 5 - 6ppm. If it's hot the chlorine will be used up pretty quickly, perhaps a few days, but if you're having cooler weather it may take longer. The best thing to do is to test the water every other day and see how much the chlorine decreases per day.

You can use a product called Thiosulfate. This is a chlorine neutralizer but I don't recommend using it. It can skew your readings. It's best to allow the chlorine to decrease naturally.

Also, be sure you're shocking the pool for the right reasons, not simply because it's Saturday. The only time you really need to shock the pool is when the combined chlorine is 0.6ppm for three consecutive days. Many pool store employees are told to tell customers to shock the pool on a weekly basis and this simply is not accurate. If you keep the chlorine between 1.5 - 3.5ppm and the CYA 30 - 50ppm there's no reason to shock the pool. The chlorine is already doing its job killing off the organic matter and bacteria.

If you would like personal assistance, I do phone consultations for a donation of your choice. It makes things go much faster. If you choose to not go that route, we can correspond by email.

Contact Me


Hope this helps.


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Jul 05, 2014
Changing Sand Filter
by: Sur

A pool store owner recently told me there is no reason to change out the sand in a sand filter at 6 years... He said 30 years! I think that is incorrect. Please help!

Thanks for the question Sur

With normal maintenance the sand can last between 5 - 10 years. 30 years is a bit extreme. After that the filtration become less effective. How to you know what is "less effective"?

First is water clarity. If the chemicals are balanced and the clarity is decreasing, it's probably time to change the sand.

2nd is to turn the system off and remove the top of the filter. The top layer of sand should be smooth all the way around and to the edges. If there's a dip at the edges some of the dirty water is probably going down the sides. This is called channeling. If this happens you can try to degrease the filter using GLB Filter Fresh. If this doesn't work AND the chems are balanced you can change the sand.

Also there's no need to change the sand before 5 years when you have the right amount of sand, proper filtration, and the chems are balanced.

Don't do something simply because it's Saturday and you have nothing else to do. Be sure there's a reason for your pool maintenance. If it ain't broke don't fix it.

Have a great Summer.


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Superchlorinating pool

I have a question about superchlorinating a pool.

How long before you can swim after superchlorinating swimming pool?


Good Question

It's not really the amount of time as it is the amount of chlorine left in the pool. There can be many factors to consider as to how long it takes to go swimming after you super-chlorinate:

1. How high the chlorine is
2. If you put a cover on the pool
3. Sun
4. Heat
5. Any organic matter that falls into the pool (leaves, grass clippings, etc..) that will use up some chlorine
6. Filtration cycle

Adjust Your Swimming Pool Chlorine..Swimming Pool Chemistry..Swimming Pool Care.

Probably a good rule of thumb is to wait 2 days after super-chlorinating your pool. Test the water using a Taylor Reagent K-2006 test kit. When the chlorine gets down between 1.5ppm - 3.5ppm your pool should be safe to swim in.

Water Testing Kit..Swimming Pool Chemistry..Swimming Pool Care

Keep your cyanuric acid level between 20 - 40.

Adjust Your Swimming Pool Chlorine..Swimming Pool Chemistry..Swimming Pool Care.

If you need to use the pool sooner you can add a chemical called Thiosulphate to reduce the chlorine faster. This can be purchased at any pool supply store.

Best regards and thanks for the question


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Total Chlorine..Free Chlorine..Pool Shock Question..

by Jose
(New York)

On a 330,00 gallon pool, it has a total chlorine of 1.3 and a free chlorine of 0.6.

I need to break point the pool how much of the following chemicals I need:
a- how many lbs calcium hypoclorite

b- how many gals of sodium hypoclorite

c- how many lbs of gas chlorine

Thanks for the question Jose

The question is talking about a desired change in the chlorine of the pool. I will assume it's a home pool with 33,000 gallon pool and not 330,000.

The general rule to raise the chlorine level up 10ppm for every 10,000 gallons of water:

Calcium Hypochlorite (67%) = 1.25 lb
Sodium Hypochlorite (12%) = 3.25 qts
Gas Chlorine = 13oz.

Taking these numbers you would need:

4.75 lbs. of calcium hypo. to reach break-point chlorination

2.75 gallons (round up to 3) for sodium hypo.

2.5 lbs. of gas chlorine

Very easy. Just multiply the 1.25 for cal hypo. by 3, which gives you 3.75 lbs. You have 3,000 gallons but you always want to over-shoot a bit when shocking a pool. 5,000 gallons would be 0.625 lbs. which is very close to 1 full lbs.

Add the 3.75 and 1 to equal 4.75. You can use the equation for liquid and gas as well.

Swimming Pool Chlorine

Pool Shock

Hope this help and good luck


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How Much Shock vs. CYA Level?

by Scott

I have a 10,000 gallon vinyl lined above ground pool with a cya level of 40. Ph is 7.8 and calcium hardness is 200 ppm. I generally keep the FC at 3.5 - 4.0.

How much sodium hypochlorite should I add to shock with my CYA level at 40? Also, is my daily FC level at 3.5 - 4.0 appropriate according to the CYA level?

Thank You


Thanks for the question Scott

The CYA/chlorine relationship is one of the easiest to understand. It's simply this: the pool chlorine is 7.5% of your CYA and the range is 30 - 50ppm. A FC of 7ppm means for it to be active enough to kill bacteria, the CYA will need to be 93. This means a water change is needed to reduce the CYA and get it in range of 30 - 50. A 2/3 drain and refill should reduce the CYA to around 30ppm.

Remember as the CYA rises through use of Trichlor tabs the chlorine must also increase to keep this relationship. You can have an increased CYA level, say 60 - 70ppm, but the chlorine level must rise as well to keep that relationship.

Normal residential pools should have the chlorine level between 1.5 - 3.5ppm. Yours is 3.5 - 4.0ppm, which is good, but perhaps a bit high. Unless you have a heavily used pool or cannot take the readings on a weekly basis, 1.5 - 3.5ppm can work for you.

Calcium isn't that important for vinyl pools. This is mostly for plaster. Calcium keeps the water from drawing the calcium carbonate out of the plaster. Keep your vinyl pool between 80 - 100ppm and you should be fine.

Sodium hypo doesn't have any stabilizer in it. It's considered an unstabilized form of chlorine. It's 3 1/4 qrts. of liquid chlorine to raise 10,000 gallons 10ppm. If your CYA is in range it should not matter what the CYA is.

Remember that your chemical readings seem to be good. Shock only when needed, not because it's Saturday. Only shock when the chloramines (combined chlorine) get to be 0.6ppm for three consecutive days. Normal pool maintenance and good readings like yours and your pool shouldn't need to be shocked. The chlorine is already doing its job of killing the algae and bacteria.

Dichlor should only be used to shock a pool when the chlorine and CYA is low, below 20ppm. This is a stabilized form of chlorine.

Swimming Pool Chlorine

Pool Chlorine

Chlorine Tablets

Pool Chlorine Tablets

If you would like personal assistance, I do phone consultations for a donation of your choice. It makes things go much faster. If you choose to not go that route, we can correspond by email.

Contact Me


Hope this helps and have a great Summer.


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