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FAQ: Pond Water Quality

By: Dr Gareth Evans - Updated: 7 Jul 2020 | comments*Discuss
Water Quality Ph Hardness Nitrogen

Although we instinctively tend to think of water as ‘bad’ when it’s green and slimy and ‘good’ if it’s crystal clear, the reality is that when it comes to water quality, it’s very difficult to tell just by looking. In nature, some of the most vibrant habitats are pretty murky, while chemical filled waterways support little life – no matter how ‘clean’ they look.

Understanding water quality and knowing how to stay on top of it is arguably the single most important factor in ensuring the health of your pond – so here’s a few answers to some of the most commonly asked questions about this vitally important topic.

What are the main factors in water quality?

The main factors in water quality are nitrogen (as ammonia, nitrite and nitrate), carbonate, dissolved oxygen, pH and hardness (calcium levels).

What are pH and hardness?

pH is a measure of acidity or alkalinity – the scale runs from 0 (very acid) to 14 (very alkali); most forms of pond life prefer the water to be around neutral (pH7). All sorts of chemicals likely to be found in and around the garden can affect the pond’s pH – including fertilisers and the run-off from concrete.

Hardness is all about dissolved minerals in the water – usually calcium or what is called ‘calcium equivalent’. Depending on the geology of the area, water naturally picks up minerals as it travels over and through the ground – if it collects a large amount, it is termed ‘hard’ while water with little dissolved mineral content is called ‘soft’. Furring in your kettle is an example of the effect of hard water.

How do I make sure my water quality is OK?

There’s only one reliable way to be sure – test and keep testing regularly. The good news is that quick and easy to use water quality kits are widely available from all the usual sources of pond supplies.

How often should I test?

It depends on how new your pond is, and whether you have a known issue with water quality – in which case you should be testing at least twice or three times a week, if not more often. For established ponds with no evident signs of problems, monthly testing should be perfectly adequate – though obviously the more often you test, the quicker you’ll spot anything that is going wrong and the sooner you can do something about it.

Is it true that poor water quality is a major health threat for fish?

It certainly is; there’s an old saying that if you look after the water, the fish will pretty much look after themselves, and there are probably fewer truer words that have ever been spoken on the subject! Something in the region of 90 per cent of all fish deaths and ill health are said to be caused by bad husbandry – and poorly maintained water quality is widely accepted as the biggest factor.

What is water ‘conditioning’ – and how do I do it?

Unless your home is one of the few in the UK fed by a private spring or borehole, the water coming out of your tap is full of all manner of chemicals which, although essential to guarantee the purity of the supply as it travels to your home, are harmful to pond life. Conditioning – also sometimes called maturing – is simply the process of allowing the water in a newly filled pond to lose the chlorine and other unwanted substances and gradually turn into something more friendly to pond plants and fish.

Why do my dissolved oxygen levels drop in the summer?

There are two reasons for this. Principally it’s down to the fact that water naturally holds less oxygen when it is warm than when it is cold, so as the temperature rises, the level drops.

At the same time, warmer conditions stimulate fish and other pond animals into greater activity – moving around, eating and excreting more – and for many this is when breeding is at its peak too, generating even more bodies to claim the dwindling amount of oxygen in the water. This is partly offset by the photosynthetic activity of the plants, but unless the pond is very large and very well-planted, it doesn’t really make enough of a contribution.

What’s the single best thing I can do to safeguard water quality?

Install the best and most efficient bio-filter and pump system that you can afford and reasonably fit into your pond and garden. In one fell swoop, once it’s established and running properly, you’ll have taken care of four of the seven most important factors in water quality – ammonia, nitrite, nitrate and dissolved oxygen – and probably made dealing with the remainder a lot easier too.

Good water quality is vital, but arm yourself with a good test kit and a bit of an understanding of what’s going on, and you should be able to avoid hitting any major problems without too much difficulty – just don’t forget to keep testing!

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Hi, my pond is 1month old now, I have an oase 10000 biopress filter with waterfall, and I have a few small koi, my water is clear but a brown colour and lots of algae growing very quickly on stones, I have tried charcoal in the water, also a pond bomb, but can't get rid of the brown water, I am in Scotland,, also the pond gets about hours of sun in the evening, many thanks, gordy,
Gordy - 7-Jul-20 @ 7:58 PM
I've made a new wildlife pond. I filled about half with tap water and the rest from a rain butt which had been lying for a while, but looked ok. I left it for over a week then put plants in. Another two weeks, plants are thriving but not a single piece of life in it. No insects , nothing. Birds bathe in it, that's it. Any ideas?
Steff - 1-May-20 @ 9:42 AM
I installed a pond in our garden 15 years ago. All was well for many years but recently plants are not surviving and there are very few water bugs. We have no fish as they went missing 5 years since. I’ve just cleaned the whole pond out for the second time in 3 years. The water was black with suspended silt which adheres to everything and chokes it.The snails are doing reasonably well but are struggling with the blanket weed which floureshes around the edge.I’d like to know what is causing this black silt. Any ideas would be welcome.
Baz - 30-Mar-19 @ 5:16 PM
I have a large pond (5m x 5m) with aprox 30 goldfish, The pond is lined with specific pond liner. My problem is my water lilies are not doing very well, before we lined the pond they were thriving. Since dividing them and repotting with the right soil, fertilizer etc they are stunted and discoloured and no flowers. What can I do ?
Kathy - 10-Jun-18 @ 11:47 PM
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