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Pond Water Quality

By: Dr Gareth Evans - Updated: 8 Oct 2018 | comments*Discuss
Fish Health Ammonia Nitrite Nitrate Ph

Water 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.


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.

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Hi Built a wildlife pond in the spring 9mx7m and 2m deep, no fish. Put lots of plants in it and oxygenators. Had lots of interest from dragon flies and beetles etc over the summer, but now it's looking sad....the plants are dying and the insects have left there is no life there at all. pH is fine about 7.5 I am guessing that for such a big pond with no fish it has run out of nitrogen to feed the plants......? They are just dying and forming a brown sludge on the bottom. Water is clear and there is no added soil. Any thoughts? Shall I just wait till next year and see what happens? Can I add nitrogen? Seems nuts when everyone needs is talking about how to reduce it. Or could it be another problem? Cheers
Wild goose - 8-Oct-18 @ 2:27 PM
I can’t seem to get my pond water clear. I’ve vacuumed the silt from the bottom, I’ve got a little string algae but not a lot, I’ve got a new uv light and backwash the pump regularly. I use microbe lift pl weekly and have done a water change out. Could it be the size of my fish? I have 3 koi that are about 15-20” long and my pond is about 500 gallons. It is a cement wall pond and I have a pop up tent over it that shades it most of the day. What am I doing wrong?
Liz - 21-Jul-18 @ 11:51 PM
my pond is new and was filled about a 2 months ago. The algie has gone but the water remains very brown, will this clear in time. It is a wild life pond and I have put oxygenters in. Any advice would be useful. Thanks, Terry Willett.
terence - 27-Apr-17 @ 3:34 PM
My 500 gallon pond was clear & fish were healthy although overstocked, The liner failed & had to fit a new one the fish were removed into a small tank.The liner fitted & filled with water, A new pump,filter cleaned I had no choice but to put the fish back in or get rid of a lot of them [There was about 40 4-6 inch & 60 1-2 inch] so they all went back in.As you would expect it went green this was in june 2015.I For a start i left the pond alone I then cleaned the top two filter media with pond water then i tried using some blagdon pond clear over the last five weeks, but no luck.I have just tested the water with some test strips.Hardness was 30ppm, Alk 80ppmPH 7.5Nitrite 0Nitrate 0 Anybody any ideas??
Shorty - 20-May-16 @ 2:32 PM
Hi. the water in my small pond started to smell. The gold fish were "gasping" at the surface. Iimproved filtration and am running a fountainThe smell persists but the fish are no longer gasping. The water is relatively clear but we experience Algae problems. The water lillies pretty much cover the surface so I hope the algae problems are not going to materialise. I am wondering wht else I can do?
Rich - 15-Jul-15 @ 10:24 AM
What range of total hardness is acceptable for goldfish in a pond. What range of pH is acceptable too.Will a large chalk rock in the pond be a good idea?
holisticjohn - 13-Sep-14 @ 4:10 PM
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