Pond Water QualityWater quality is the single most important factor in ensuring a healthy pond – particularly if it is stocked with fish – but quality is not the same as clarity. Just because water appears crystal clear and sparkling it does not guarantee that it is good for aquatic life any more than being green and murky – however unsightly that may be – makes it automatically bad. When it comes to water quality, you are not going to get any clues simply by looking; a little bit of science is what you need and the good news is that with today’s readily available and decidedly user-friendly test kits, it has never been easier to do.

What to Watch

There are seven main things to keep an eye on – ammonia, nitrite, nitrate, dissolved oxygen, carbonate, pH and water hardness.

The first three on the list – ammonia, nitrites and nitrates – are all nitrogen-containing chemicals and form part of a linked natural cycle, one sort of bacteria breaking down ammonia to form nitrites, which another kind subsequently makes into nitrates. Fish turn around five per cent of the food they eat into ammonia which they then excrete. All three of these nitrogenous chemicals are toxic and if they are allowed to build-up in the pond, will cause serious health problems for the fish – the ideal ammonia and nitrite levels need to be as near zero as possible and nitrate no more than 50 parts per million (ppm). A good bio-filter can do this, making use of resident bacteria to make nitrite and then nitrate – but, although nitrate is the least toxic of these nitrogenous compounds, it is an excellent plant nutrient and excess can feed algal blooms.

Oxygen dissolves naturally in water, though the amount held varies with temperature – the warmer the water, the less it contains, which makes the possibility of low oxygen levels a potential summer problem – precisely when fish require more, rather than less. A good fountain, waterfall or cascade arrangement should help ensure that sufficient is added to meet their needs, but do remember that although plants contribute abundant oxygen to the water during daylight, they also use it at night. If the pond is heavily planted – or green with algae – the combined drain of fish and plants may deplete oxygen levels as dawn approaches, especially if the previous day warmed up the pond.

Different species of fish have their own particular needs when it comes to pH and water hardness – and sometimes the limits can be fairly narrow – and it is obviously important to be sure that the water in any pond suits the specific requirements of its inhabitants. Testing the carbonate levels is helpful because it gives an indication of the stability of the pH; carbonate and bicarbonate ions act as buffers, stopping the acid/alkali balance from shifting too quickly – and fluctuating pH does no good at all to fish of any kind.

Testing

Water quality problems are most likely in a young pond, so testing should be particularly rigorous after the first stocking and throughout the following three or four months, especially if any further fish are to be added. However, even mature ponds can suffer during the spring, as things begin to wake up again – but at different rates – sometimes leading to a short term imbalance, so intensive testing early in the year can also be something to consider if your pond seems to be affected. Once things have settled down, fortnightly or monthly checks may be all that is required even for serious koi ponds – and even less frequently for others. Some things – notably pH and dissolved oxygen – fluctuate naturally over the day, so always try to test at the same time so you can make useful comparisons between your results – which should always be written down. You never know when you might want to refer back to them.

Only with a regular testing regime can problems be spotted before they escalate into a serious threat to the health of your pond and enable appropriate action to be taken. Often remedying the problem is fairly simple if caught early enough, such as cutting back on feeding if any of the nitrogen readings are too high, adding some limestone – a good source of carbonate – to buffer a fluctuating pH or turning up the pump if the oxygen is too low.

It is often said that if you look after the water, the fish will look after themselves, which is a pretty fair comment when you consider that the majority of the health problems – and deaths – in fishponds can be attributed to poor water quality. While it may not be so critical to get the chemistry just as spot-on for wildlife or plants, for instance, as it is for koi, the same principle applies; healthy ponds need good water quality – the science is as simple as that.