If you dig a hole in your garden, line it to make it water tight and fill it with nice clean tap water what have you got? A swimming pool or a pond? Well, it depends what you do next. Leave it and it will soon fill up with algae blown in from the atmosphere and turn green. Bacteria will grow in it and, well, you wouldn't want to swim in it.

But dissolve in a small amount of the right chemicals and algae will be killed as soon as it drops in and bacteria will not only be killed but destroyed. Filter out the small particles and you have a fine swimming pool that anyone and everyone will want to swim in. Its easy!

There are basically three types of chemicals you need to keep your pool water safe and clear:

• Chemicals to kill stuff.
• Chemicals to clear away the dead bodies.
• Chemicals to make the other two work properly.

The most common chemical used to kill stuff is chlorine. No one messes with chlorine. If you are a bacteria chlorine is your worst nightmare, it will kill you and all your family in less than two seconds and once you're dead it will chop up your body to bits so small even an electron microscope can't see. Not just bacteria, algae, skin cells - anything organic.

Though it is quick to kill it takes a while to chop up so that's why we filter the water to get out the dead bodies and the filter sometimes needs help with the really small bits so floculants are added to gather all the small bits to one big bit that the filter can catch.

But in order for the chlorine and floc to work properly the water must be 'in balance' that is the acidity level must be right. Dry Acid or Alkali needs to be added to get the balance right.

And that's it in a nutshell, the difference between a pond and a pool.