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Water Quality Management and Water. Garden Fish Pond pH and Hardness

pH and the Pond

In a swimming pool if you measure the pH at noon and again at midnight it will be the same. In a pond with plants it will not be the same because of the photosynthesis cycle. During the night the pH will fall before rising again during the day to its original level. With high levels of algae the pH can change dramatically with dire consequences.

pH is a measure of the acidity or alkalinity of water. The measurement scale is from 0 to 14. At the mid point of the range the water is neutral since the acid and alkali balance each other. This is the case for pure water.

Less than 7 the water is acidic (vinegar is less then 7, a lemon is about 2.2). More than 7 and the water is alkaline (sodium carbonate is a good example and milk is between 7.1 and 8.5). Stable levels of pH are desirable and a range between 7 and 8.0 is good for a pond. When you start seeing levels of 9 there is a danger developing.

pH, even though it is simple to measure (but not accurately) is a highly complex phenomenon. pH has a great influence on almost everything in a pond environment. It effects how the biofilter works, it affects the fish, it in turn is affected by the addition or removal of pond water and whether the water from the tap in your region is hard or soft. At higher pH levels toxic levels of ammonia become deadly.

To have a high level of control over pH fluctuation it is important to have a good level of hardness in the pond. This is called buffering.

pH and Water Hardness ... What is this hardness?

When you wash your hands in water and you find it difficult to get a good lather (even using quality soaps) you are in an area where the water is naturally hard. Conversely when the water feels slimy when you wash your hands the water is called soft. High levels of dissolved salts and particularly magnesium and calcium salts cause hardness in water.

There are also 2 types of hardness - permanent hardness and temporary hardness.

  1. Water that has permanent hardness has a lot of magnesium and/or calcium SULPHATE salts as well as other salts.

  2. Temporary hardness is due to magnesium and/or calcium CARBONATES. If you look inside your kettle you might see deposits of white or grey solids on the heating element. This is CARBONATE coming out of the water when it is boiled and this is why it is called temporary. CARBONATE hardness is important in a pond in order to control pH.

The total of the SULPHATES and CARBONATES in water is called permanent hardness or TH for short. The amount of CARBONATES only is called temporary hardness and is referred to as CH for short. Some countries use different terminologies including DH and KH.

Total hardness (TH) is important for the good physiology of fish.

The following is what happens in a pond that has a low level of CARBONATES.

During the day any plants (and this includes algae which is a plant) in the water remove this small amount of carbonate and as a result the pH of the water can climb significantly and maybe even reach a level of 9.

At this level ammonia secreted by the fish themselves is EXTREMELY poisonous to fish and your fish can die - under these conditions your fish will appear distressed, gasping for air, lying on their side, lethargic and so on.

Now if there were higher levels of CH in a pond it would be difficult for the pH to rise to 9 and it would also remain a lot more stable, which is much better for fish.

If you suspect ammonia poisoning then immediately test the water for ammonia if you have the kit. If you are unable to test or if the test proves positive start changing some of the pond water immediately (say 50%) and DO NOT feed the fish - fish can live happily without feeding for days on end. Keep changing portions of pond water until you are able to stabilise the situation. Keep measuring the ammonia levels in the water.

If you ever go to a koi show you will see water from the holding tanks being changed frequently to prevent ammonia build-up.

Think of CH as preventing pH from fluctuating widely and reaching dangerously high levels. This is good for fish and plants in a pond. That is all you need to remember.

  • Good levels of TH in a pond are 7 to 14 degrees

  • Good CH levels in ponds are 6 to 12 degrees

The only way you know if you have the correct levels is to test the water yourself by buying a suitable test kit or take a water sample to a fish outlet with these testing facilities. If you find the hardness is too low then select a product to correct the problem and follow the instructions carefully. Hardness can be increased by using dolomite, marble chips or crushed shells.

It is also a good idea to test your tap water to see if it is hard or soft so you can forecast what might happen over a period of time.

Acceptable Values For Pond Testing

  •  Ammonia: Nil

  •  Nitrite: less than 0.3 mg/litre

  •  Nitrate: 25-100 mg/litre

  •  pH: 7.0 - 8.0

  •  TH: 7 14 deg TH

  •  CH: 6 - 12 deg CH


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