What is Wrong with my Pond Water?

Usually at the opposite ends of the year, winter and summer, ponds around the UK take a turn for the worse. This is often due to changes in the weather and the pond’s environment, and like with everything biological, early detection and treatment is the best way forward.

Here is a list of the 4 things that could be wrong with your pond water:


If you have taken a stroll down through the garden to find a pond that resembles a horror-film swamp, it could be that your nitrate levels are too high.

Nitrates are used on farms as fertiliser as they are absorbed by vegetation and used in photosynthesis, and the algae in your pond is no different – it uses nitrogen in the nitrates to feed, so a lot of algae is often a good indicator of its’ presence, either naturally forming nitrates or ones that have leached into your pond from nearby use of fertiliser (even garden compost).

You can test for nitrates using nitrate test kits (which are widely available) even before your pond turns to mush. It’s good to know what is in your pond so you can take swift action before the real symptoms like large algae blooms set it, treating the water with an algae buster.


Ammonia and nitrate levels often go hand-in-hand and you can test for it using an ammonia test kit, just like with nitrates. Ammonia levels increase when you have a lot of decaying organic matter in your pond, using up oxygen while they decompose and producing ammonia levels that once again tend to produce algae blooms.

If your ammonia levels are beginning to test high then it might be time for a bit of a pond clear out. Use a sludge buster or a pond vac to eliminate decaying matter, relieving your pond life from the ammonia increasing effects of the dead.

pH Levels

Your pond water’s pH levels should be somewhere between 6.5 and 9 (at the absolute outside figure for survival) for most Koi ponds to thrive. If they are too far outside of those margins, both your fish and your planted vegetation will begin to suffer. High pH is often a symptom of having algae in your pond, so keeping on top of your ammonia and nitrate levels should stave off the worst of what pH levels can do to your pond, but if you find that your pH levels are constantly changing, you can use some pH buffer to bring it either up or down.

Try not to make drastic changes unless completely necessary, as sudden fluctuations alter the chemistry of your pond so quickly that your fish have no time to adapt, and may die.


Water hardness refers to the level of minerals in your pond water, primarily magnesium and calcium. If your water is too hard then you are likely to get a lot of algae again, but too low and you might find that you struggle to grow any pond vegetation at all!

Irritatingly, not only does algae love harder water, but so does your koi, meaning that if your water is going to be kept hard for your koi, you need to get a good filtration system to handle the probably increase in algae.

You can get magnesium and calcium test kits easily and cheaply to keep on top of this, and if you really want to monitor your pond quality you can get all-in-one kits to help you solve your pond water problems before they begin to develop symptoms like algae blooms, vegetation death or fish loss.

If your fish seem to be suffering for any other reason, it might not actually be the water quality that is affecting it: Think about aeration of the pond and parasitic diseases, in which case you are best consulting a fish-health guide, but keeping your pond water in good condition is actually a good preventative measure for keeping disease at bay too.

Good luck getting things back to normal!


  • Michael Marchant Posted 28/07/2014 at 4:17 pm | Permalink | Reply

    My problem is having given my pond a complete clean about 10 weeks ago its as green as green, and I have installed an extra biological filter to no avail.
    Can anyone advise me.

  • Chris Hamilton Posted 10/09/2014 at 8:58 pm | Permalink | Reply

    3weeks ago I cleaned my pond out ready to put in some new fish.I refilled the pond and used all the chemicals needed to give the right conditions and introduced the ten 4″ goldfish.Everything ok until a week ago when the water started to turn green. I tested ph and found it was to high,tested ammonia which also was high.Since then I have tried ph balancer and ammonia reducing products with no success.On testing tonight the ph is 9.5 and the ammonia level has increased. Whatever is happening? The pond is fibreglass 1130 litre and I use a bioforce 4500 with a uv light. Help!!!

  • Olive Edmunds Posted 26/04/2020 at 6:38 pm | Permalink | Reply

    can I fill a new wildlife pond with tap water

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