Chemicals & Testing

Chlorine tablets on the side of a hot tub.

All About Chlorine!

Used properly chlorine can be your best friend in maintaining hot tub water quality.  We have compiled a list of the most Frequently Asked Questions about chlorine to help that friendship progress a little more smoothly.

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Chlorine tablets and granules for hot tubs.

Should I use chlorine granules or chlorine tablets in my hot tub?

Chlorine is an essential part of your tool kit for maintaining a healthy and enjoyable spa.  As a hot tub owner, you’re probably already aware that chlorine can be added to your hot tub water in two different forms:

    • Chlorine Tablets
    • Chlorine Granules

But you may have wondered, does it make a difference whether I use chlorine tablets or chlorine granules in my hot tub?  Does one form of chlorine work better than the other?  Let’s find out!

Should I use Chlorine Granules in My Hot Tub?

The benefit of using chlorine granules is that they are quick to dissolve and relatively pH neutral.  This makes chlorine granules ideal for shock-dosing on initial set-up with fresh water, or if the spa hasn’t been used for a period of time.  Chlorine Granules will raise chlorine levels more rapidly than chlorine tablets.

An image of 1kg Relax Stabilised Chlorine Granules
An Image of Relax 1kg Chlorine Tablets Small

Should I use Chlorine Tablets in my hot tub?

Chlorine tablets offer a more “hands-off” approach to maintaining your spa water.  Chlorine tablets release chlorine slowly, making it easy to maintain a consistent level of chlorine in the water over a longer period of time.  If you are going away for a few days or have limited time to sort out the water chemical levels, chlorine tablets are a good choice for you.

Is it easier to use chlorine tablets or chlorine granules in my hot tub?

It really depends on how you use the tub!  If you don’t use your hot tub very regularly and need to shock dose often, chlorine granules may be more convenient for you.  If you want to maintain a consistent chlorine level you will need to schedule adding granules to your spa – this may become a hassle for you.  Regular spa users may find tablets more convenient.  Getting the correct chlorine levels set initially may take some experimentation, but after that, as long as you make sure that you have replaced the tablets once they have dissolved away they are fairly hassle free.

Adding Chlorine granules to a hot tub

How do I add Chlorine Granules to my hot tub?

You can add chlorine granules to your spa water simply by sprinkling them directly into the water (as long as the water temperature is greater than 20° C).    However, this method does carry a risk!  If the granules settle on the acrylic surface of your spa and do not dissolve this may cause damage which is very unlikely to be covered by the manufacturers warranty.


If the water temperature is less than 20° C, if you are unsure of the water temperature, or if you want to avoid potential damage to acrylic surfaces, then the best method is to pre-dissolve the chlorine granules in a clean plastic bucket.  Never add water to the chlorine, fill the bucket with water first then add the chlorine.

Add the dissolved Chlorine Granules when the pumps are on, and near to the water inlets as this will aid the distribution of chlorine around your spa.

Don’t get carried away!  You don’t want to over-chlorinate your water.  So just add one dose at a time.

How much Chlorine Granules should I add to my hot tub?

A little maths is required!

2g of Chlorine Granules will increase the chlorine level of 1,000 litres of water by 1mg/l (ppm).

If you are pre- dissolving your granules in a bucket of water you can follow this formula up to a maximum rate of 10g Chlorine to 1 litre of water.  To be accurate you will need to know the volume of water in your hot tub.  Alternatively, you can add a little chlorine at a time, allow it to circulate and then test and re-test until you achieve the chlorine level you are aiming for.

Chlorine  Dispenser for 20g Tablets

How do I add Chlorine Tablets to my hot tub?

The amount of chlorine released is controlled by adjusting the dial of a floating chemical dispenser.  A little bit of trial and error might be needed at first to find the sweet spot on your chemical floater that maintains the desired levels of chlorine. However, once you have found the level that works for you it should be relatively easy to keep chlorine levels consistent.

How often do I need to replace chlorine tablets?

Chlorine tablets will typically erode over a period of 3-5 days, depending on the amount of water flowing over them.

How do I know if I have added enough chlorine to my hot tub?

Before bathing, you should check the chlorine level is within the range of 3-5mg / l (ppm).  You can check your free chlorine levels with a test kit.


Help!  I’m allergic to chlorine!

A lot of people think they are allergic or intolerant to chlorine.  For most people this isn’t the case.  Water pH levels play a huge role in how comfortable we are in water.  Water that is either too acidic or too alkaline will irritate our skin, eyes and sinuses.  The water pH must be balanced and neutral for water that is comfortable and also for the sanitiser to work effectively.

3 Way Chlorine Test Strips
Aquafinesse Hot Tub & Spa Water Care System Switch Kit

How Can I Use less chlorine in my hot tub?

High levels of chlorine can be undesirable.  The keys to maintaining safe and healthy hot tub water while using lower levels of chlorine as a sanitiser are:

    • Keeping pH levels balanced – ideally close to pH 7.2
    • Use AquaFinesse to eliminate bio-film

At a pH of 8, over half of chlorine added to your hot tub water is ineffective.  At a pH of 7.2 this rises to 90% – chlorine which is actively available to kill algae and bacteria.

Biofilm, a type of slim can form which provides a safe shelter for bacteria allowing them to breed virtually unmolested by chlorine or any other sanitiser.  AquaFinesse breaks this shelter down allowing smaller amounts of chlorine to be more effective.

My hot tub water smells strongly of chlorine what should I do?

When chlorine combines with organic waste (body fluids, dead skin etc) it produces a compound called chloramine.  This gives off a strong chlorine odour.  This is often mistaken for too much chlorine but is in fact an indication that the chlorine has been “used” up.  A shock treatment will breakdown chloramine so it can be easily removed from the water, “freeing up” your chlorine to work more effectively.

Do I have to use chlorine in my hot tub?

No.  You don’t have to use chlorine.  But you do have to use a water sanitiser.  For most people chlorine is the safest and most effective sanitiser to use in their spa.  An alternative sanitiser is bromine.

We hope you have found our guide to using chlorine in your hot tub or spa useful.  Check back regular as we add to our Water Quality FAQs!

An image of a 1kg bottle of Relax Bromine Granules Hot Tub / Spa Sanitizer

Hot Tub Super Tips!

Hot Tub Super Tips!


hot tub gazebo

Do you own a hot tub or spa? Then this is for you, quick and easy tips to help you maintain your hot tub and keep it in tip top running condition:




Keep hot tub filters clean

It is important to keep your filters nice and clean as they are an important part of the hot tub water system and the water is constantly ‘recycled’ through them.

Hot tub filters deal with body oils/fats that’s are lost through bathing and also scoop any organic debris out of the water, so using a good filter cleaning product is important


Get back up filters

Rotating your filters is always a great idea as you always have a spare set plus you will find it can help the filters to last a lot longer.

We have heard of many people using various methods of filter cleaning including washing machines(this is not recommended)! We recommend maintaining the integrity of the filter by using

AquaFinesse Filter Cleaner, Aquasparkle filter cleaning spray or Immerse Filter cleaning cartridges

If you would like advice on how to clean your spa filters please contact a member of our knowledgeable staff


ph minus 

The balance of pH in water is imperative to the comfort of your bathers and also the efficiency of your sanitiser.

Testing strips will soon become your best friends in the pH balancing game. Keeping the hot tub water pH balanced will have a knock on effect on the rest of your water balancing and you should find it all becomes much easier to manage!






hot tub chemicals testingAlways avoid household chemicals when cleaning your hot tubs, we again often hear of people using bleach or cleaning sprays on their hot tubs

*Please only use hot tub safe chemicals and cleaners! Our website has a comprehensive range here:


Here you will find hot tub shell cleaners, chlorine tablets and granules, bromine tablets and granules, flocculant, *Use caution when adding spa chemicals*



hot tub repairIt is important to regularly deep clean your hot tub with a biofilm eliminator; this will flush your pipework properly and ensure safe bathing water. A Grit gitter or spa vacuum are handy tools for in between deep cleans.We have written a very informative blog on how to deep clean your hot tub here!


We always advise spa servicing by hot tub engineers who will do a 5 point hot tub check and ensure all of your heating elements pumps and seals etc are working properly, your car is MOT’d regularly and your hot tub needs the same treatment.

Our Spa & hot tub parts section has everything you need if you need hot tub spare parts i.e. pumps

Schedule of hot tub maintenance:


Test and balance water with spa testing strips

Remove hot tub filters thoroughly clean & rinse then leave to dry

Shock spa water

Wipe down the underside of your cover to prevent stale smells or mould forming!


Clean spa cabinet and wipe spa cover down we recommend using 303 Aerospace Protectant for longevity

Deep clean your spa using a biofilm eliminator


Annual service by a spa engineer


Take advantage of ‘off peak’ heating costs- set your spa to heat in the evening when electricity costs are lower

Watch water temp in winter do not let it freeze – as it can cause damage!

deep clean

Which Hot Tub Deep Clean Product Is Best?

Which Hot Tub Deep Clean Product Is Best?


I want to deep clean my hot tub but can’t decide which of your hot tub cleaning products to choose…..


There are two giants in the deep cleaning department for hot tubs & swim spas the first one is the AQUAFINESSE SPA CLEAN TABLET


Aquafinesse Spa Clean Tablet


This is an environmentally friendly gentle deep cleaner, it will thoroughly clean out the pipe work in your spa before you empty it and refresh the whole hot tub ready for a refill. As this is a gentle cleaner you can expect it to take up to 24 hours to complete.


The second one is the AHH-SOME HOT TUB CLEANER this more potent hot tub cleaner will do the exact same job as the AquaFinesse one.



The pipe work and hot tub will be thoroughly cleaned and refreshed within 1 -2 hours

The best advice is to give us a call on

T: 01348 840509 to discuss any deep cleaning questions you may have with a member of our knowledgeable staff who will be happy to help you.


Waterline Cleaning Paste - Castle Hot Tubs

Scum On Your Hot Tub Waterline? We Have The Solution!

Scum On Your Hot Tub Waterline?  We Have The Solution!

Wondering how to clean your hot tub scum line properly?

Facing the job of cleaning that stubborn scummy line of body fats, oils and general dirt off your spa can be a daunting task!

Many of our customers call us to ask how to clean the scum build up off their hot tub/spa waterline as it is notoriously hard to remove and traditional cleaners don’t work well with your hot tub water.

Don’t make the mistake of using cleaners not suitable for use with hot tubs/spas as they can cause foams and your hot tub water chemicals to imbalance.

Keeping the balance of your hot tub water chemicals right is very important, if the chemicals don’t balance right in your spa then you are looking at anything from an uncomfortable to a dangerous bathing environment in your hot tub for bathers.



Waterline Cleaning Paste - Castle Hot Tubs

The BLUE HORIZONS WATERLINE CLEANING PASTE is a highly effective and simple way to remove the grime caused by body oils, fats and greases at the spa waterline.

Easy to use, it works in under 5 minutes and takes the stress out of cleaning the scum and grime off your hot tub.

Simply lower the water in your spa and use the waterline cleaning paste as per instructions.

You can purchase waterline cleaning paste here, with free UK Mainland delivery

Alternatively call our team on

T: 01348 840509




hot tub start up guide

Easy Hot Tub Start Up Guide

Have you just bought a new Hot Tub and have no idea on how to start using it? Or perhaps you have just emptied it for the first time and would like to know how to refill it correctly. We have put together this simple info-graphic guide on the basic steps to starting up your Hot Tub.

If you are ever unsure of how to use your water safely please remember don’t take risks. Call, email or use our live chat feature to contact a member of the Castle Hot Tubs Team and we will happily assist any questions you may have!

Hot Tub Set Up Guide
Our simple guide on how to set up your hot tub properly!

How To Clean Your Hot Tub Filters - Video

Cleaning your Hot Tub filters is an essential part of your regular Hot Tub maintenance routine.

Your spa  filters play an important role in keeping your hot tub water clean and free of body oils and particles that may come off the body when bathing.


It is advised that you clean your filter cartridges once a week letting them soak in a proper spa  filter cleaning solution of your choice. It is recommended that you change your filters every 12 months, however  your filters can last between 18 months and two years if looked after properly. We personally recommend having your filters on a rotational system whereby whilst cleaning your filters you have a spare set to put into your tub so the others may dry completely before the next are ready to be cleaned.


To help you with the process involved with cleaning your filters we have made this quick demonstration video of how you should be cleaning your filters.

Castle Hot Tubs sell many, many different kinds of hot tub filters to find yours, please view our shop.

If you are ever unsure of your filter type or size please call our knowledgeable  team and we will happily assist you :


01348 840509.

There are a number of filter cleaning products available on the market. To view them and find the one most suited to you, please view our shop or call us.