Pond pH keeps changing

Koi benefit significantly from stable water conditions, where water quality parameters such as temperature, dissolved oxygen, ammonia and nitrite should be maintained within desirable levels. This is especially true for pH which must be remain consistently between pH7 and pH9

pH is one of the most widely used and quoted tests and should regularly show a slightly alkaline pH. However, several factors can cause the pH to fluctuate either side of this desirable band with predictable adverse effects on koi health. If pH levels do tend to fluctuate wildly, it is critical for the long-term health of your koi that you try to identify some of the likely causes of these detrimental swings with a view to preventing them in the future.

1. What if my pH is greater than 9.0?

This stressful highly alkaline water can cause fish to gasp at the surface, and if experienced long term, will stimulate fish to secrete excessive amounts of a milky protective film of mucus on the skin.

A likely cause of a high pH is the introduction of a highly alkaline substance such as run-off from new or untreated concrete or cement work. Builder’s lime (calcium hydroxide) is an extremely caustic substance, and if left exposed or untreated on brickwork around the pond may cause the pH to rise rapidly.

There are two ways of preventing this problem from happening.

A. Neutralise the lime. Neutralising agents can be added to the cement or concrete during mixing. This reduces the hazard of alkaline lime running into the pond.

B. Paint all concrete/cement work with a sealant. A very effective, easy to use transparent and colourless sealant such as G4 can be painted onto surrounding rockwork. If used on a waterfall, it has the added benefit of waterproofing the feature. As the pond water does not come into contact with the alkaline lime, then the pH of the pond water will remain unaffected.

2. What if my pH drops below 7.0 and becomes acidic.

If pond water has the chance of becoming acidic then problems can accelerate very quickly within the pond and lead to dramatic changes in koi health. It is interesting to note that the symptoms shown by koi suffering from a low pH are very similar to those when experiencing a very alkaline pH.

The most likely cause of a low pH is the neglect of water changes. The overall effect of biological processes such as respiration and particularly biofiltration is to cause the pH to drop. As ammonia (NH3) is broken down into nitrite (NO2) by bacteria in the biofilter, the hydrogen ions released acidify the water, and as these accumulate, the pH drops further.

This phenomenon takes place continuously and will cause the pH to drop with time.

Preventing a drop in pH.

There are two ways, if used together, that will prevent an unhealthy drop in pH and help to maintain a better overall water quality.

A. Addition of a buffering material.

By adding a net-bag of crushed shell or limestone chips to a filter, the pH can easily be prevented from dropping below pH7. The calcium carbonate in these buffers will ‘mop up’ the acid-causing hydrogen ions, thereby preventing the pH from dropping. A simple, cheap and effective means of maintaining a stable pH. ‘Every pond should have some’.

B. Regular partial water changes.

These should be carried out to dilute the build up of persistent by products of fish metabolism and biofiltration. A partial water change will reduce nitrates and as most tapwater is buffered by water companies to become alkaline, will automatically add more stability to the pH.

NB. A number of koi keepers prefer to top up their ponds with rainwater to avoid the introduction of toxins such as chlorine and chloramine. Unfortunately rainwater is naturally slightly acidic and does not contain a natural buffer. If too much rainwater is added to a pond then this will increase its tendency to become unhealthily acidic.

Kill blanketweed and string algae.