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Could Barley Straw be the Cause of My Red Pond?

By: Dr Gareth Evans - Updated: 10 Apr 2020 | comments*Discuss
Pond Algae Water Quality Bacteria Uv


I have had my pond now nearly 3 months, I have cracked the water quality issue, well that's what the tests tell me. The pond is not green anymore, however, it has started to go a redish colour, the pond walls are red too with what looks like dead algae, I have barley straw in the pond should I take this out?

(P.J, 25 June 2009)


I’m sorry that you’re having this problem with what I assume is a new pond.

Obviously it’s a little difficult to be sure without knowing a bit more about the history (it sounds like you’ve had some water quality problems) the dimensions of the pond, its location, how it’s stocked and so on – but hopefully a few suggestions will help you sort things out. You’re probably going to have to do a bit of CSI-type work to figure out what’s going on.

Barley Straw

Barley straw has a long history of use to control algae in ponds and there’s plenty of evidence to back up its effectiveness and the Centre for Aquatic Plant Management has looked into the science behind it. There are no real side-effects of using it correctly – although it can cause the water to become deoxygenated if the recommended dose rates are greatly exceeded. Between 25 and 50 grams of straw per square metre of pond surface is an ideal amount, but provided you haven’t been throwing in something like ten times as much, it seems highly unlikely that barley straw’s the problem.

Algae, Minerals and Bacteria

There are red algae – properly known as Rhytophyta – but most of these are marine species, although some kinds are found in freshwater. It doesn’t, however, sound like that is what’s affecting your pond since they tend to attach themselves to plant leaves and other suitable surfaces, growing into a distinct beard-like fuzz – they’re sometimes called “beard algae” as a result.

On balance, there seem to be two likely candidates for causing what you’re experiencing –mineral staining or bacteria. Excessive iron or manganese in the water might cause some or all of the results you’re seeing – and if your water tests previously showed elevated levels of either of these, then that would certainly be something to explore further.

However, I suspect that you may be suffering from one of the Serratia bacteria – most probably Serratia marcescens. Widely present in nature, these bacteria thrive in any wet or damp environment and spread in the air; a pond rich in phosphorus and other nutrients, and with well conditioned, un-chlorinated water unfortunately makes an ideal home. If your pond was previously very full of algae, their die-back may have released a spike of nutrients into the water and provided a bonanza opportunity for bacteria to colonise and grow.

For the Future

There are a few things you can do to help avoid these and other problems in the future. You might want to consider installing a good UV lamp and bio-filter – I’m assuming since you’re using barley straw, you don’t presently have either.

Another important aspect of algal control involves regulating the amount of light and nutrients available for algae to grow, especially in the early part of the summer. Making sure that your pond has adequate surface cover, being careful with fertilisers around the pond margin and keeping as much of the dead leaves and other organic matter out of the water as you can will all help keep the scourge of green water at bay.

Keep up the water testing too; many pond-keepers don’t really bother, or do it in a rather hap-hazard way, but as you quite rightly realise yourself, it’s the only way to be sure the most critical part of the pond environment stays in good health.

Best of luck with dealing with your problem; hopefully you’ll soon have a pond to be proud of!

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Hello, I have got green algae growing on all pond sides, which looks unsightly.The water is very clear, stocked with plants and the fish look healthy. Any advice please? Many thanks Melanie
Patto - 10-Apr-20 @ 12:45 PM
Thank you very much for your excellent article. We have several lined ponds, the largest of which holds about 44,000 litres and is about 3’ deep. This pond has the red staining problem described in your article. It is not mineral staining but is most likely caused by the Serratia bacteria. The pond has thrived for about 20 years and has/had about 300/400 fish in it, mainly goldfish. Last September we became fed up with the green water (which was also red-tinged) so we dredged a lot of leaves and other waste material out of the pond (we had previously done this annually but had let it go in recent years) and applied a treatment of Eco-Sure Pond Clear Aqua Plus in full accordance with the instructions. This appeared to have no effect and when I contacted the supplier they kindly gave me another kit which I applied some 6 weeks later. Now, as the temperature has warmed up, the red bloom has exploded and formed a thick crust. It looks like the Eco-Sure Pond Clear was very effective in killing the algae but that this in turn has given us another problem. Clearly, our pond management has not been very good in recent years and we have allowed a lot of waste material to build up in the bottom which, together with the dying algae, is feeding the Serratia bacteria as you suggested in your article. We have now cleared all of the waste material from the bottom of the pond using a combination of nets and a Pond-vac and are using two bubble aerators. We have re-potted the water lilies and put in floating weed to try to shade the water. I shall install a bigger pond pump/fountain very soon. However, the pond is still very red and the bacteria are multiplying. Also, worryingly, I have not seen a single fish but nor have I recovered any fish bodies/bones from the mud. Can you please suggest anything else that I ought to do or is it just a matter of waiting? Is the Serratia bacteria toxic to fish, other animals or humans?
Fish Fan - 22-Apr-14 @ 12:35 PM
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