Green Water: algae in koi & goldfish ponds

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Page Summary:  Just like you and I koi or pond fish of any kind we dont like to move house. Over the years we become comfortable with our living environment. Everything is in its right place and we know where to go to get food. You can imagine that koi and goldfish in a trapped environment know the layout of the pond very well. They know where oxygen levels are best, where natural organisms congregate and thus a source of food. The koi or goldfish have also decided which parts of the pond they like best... eg just beneath the waterfall. Now you want to completely upset the koi and goldfish by first of all catching them and then subjecting them to a very stressful removal situation before introducing them to a foreign aquatic environment which is not the same at all as the environment they were used to. So take extreme care when moving your pond fish. UV is used to control pond algae

Murky Green Pond Water

Algae is a microscopic plant that forms naturally in water. It will form in swimming pools, garden fish ponds, and even in dog's bowls that are filled with water and left in a warm place.

But while it quickly becomes a problem in swimming pools, and should undoubtedly be scrubbed from the surface of animal bowls, it is not necessarily a problem if algae forms and flourishes in koi and goldfish ponds.

The real problem with algae in a fish pond is that the owner cant see the fish even if they are gold or multi-coloured koi.

There are various different types of algae, but what they all have in common is that they make the water cloudy and they grow incredibly quickly.

Most algae are green or brown in colour. If your pond is cloudy it is almost certainly due to suspended algae.

Why humans don't like algae

While koi and goldfish are happy to munch on algae, it spoils the aesthetic effect of the pond for us!

After all you can't see anything very much if the water looks like pea soup, or if there is a blanket of algae weed on the surface of the pond...

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What makes algae flourish

Algae thrives in summer, especially when the days are long and hot. In the cold winter months, on the other hand, the algae will die off and the water will clear naturally. But the problem is that because they are such a simple, quick-growing primitive plants, just a few dormant cells will quickly bloom as soon as the conditions change and it becomes hot again.

Because algae flourishes in warm conditions, it also stands to reason that it will do best in shallow ponds, because the water warms up more rapidly. But even a deep fishpond can become the perfect place for algae to grow and thrive. And of course, the deeper the pond, potentially the greater the growth may be.

Of course it isn't only heat that algae need, they also need nourishment, and this comes in the form of nutrients like nitrates and phosphates. Both of these are readily available in any fish pond water that is not carefully controlled.

Controlling algae in our fishponds

It is reasonably easy to control algae in garden ponds, depending largely on whether the water in the pond is still (with no movement), or if the water is recirculated using a pump.

Steps for still ponds include:

  • shading the pond to keep it out of direct sunlight,
  • using chemicals, specifically algaecides that will kill off algae and blanket weed,
  • planting pond plants that take over from algae and establish themselves within the water,
  • putting bales of barley straw or oat hay (or straw) into the water that gradually breaks down, and effectively gets rid of algae, and
  • adding a beneficial bacteria to the water that will get rid of the nutrients that the algae need to thrive.

The algae in recirculating ponds can be controlled by installing an Ultra Violet (UV) clarifier – frequently referred to as an UVc. These units have become very popular in recent years as they are very effective and easy to install. Used in conjunction with either a biological or mechanical filter, a UVc will kill the algae (and also bacteria and various fish parasites), and clear the cloudy water in your koi pond.


Don't stress over pond pH and Testing Koi Pond pH

Interpet Koi Pond Test Kit

I will make a few very important points here which should be taken very seriously by anyone contemplating measuring pH in a koi pond and then TAKING ACTION based upon the measured result from testing the koi pond's water. If you test do it reliably... Interpet lead this pond water testing field of the koi keeping hobby

  1. pH of koi pond water will vary depending on time of day measurement is taken and especially if plants are in the pond. This means if you take a pH at 9am it will not be the same as a pH taken at 6pm. This single piece of information therefore must never be the information relied upon to make fundamental decisions
  2. pH is notoriously difficult to measure even in a laboratory with sophisticated instruments let alone in a koipond or fish pond using a piece of litmus type paper or a vial with coloured scales on it.
  3. If you must measure pond pH and this is a great part of the hobby then take the pond pH at the same time every day and plot the result on a graph. So now what you're looking for is not a single pondwater result but rather a pH trend in the pond water that you can use to sensibly consider a pH strategy.
  4. Very high pH in a koi pond is a serious threat to the well being of koi when levels reach about 9.5 because at this pH level ammonia which is a natural metabolic product of the koi and its existence becomes extremely poisonous
  5. It would be extremely rare for a koi pond to reach dangerous pH levels but could well occur in koi ponds with serious algae problems

My Very Best Pond pH Advice

Consider the testing of pH as a way of discovering the normal and natural variations that exist in every pond day in and day out. Tale a look at what happens in mid summer and the cold depths of winter. See testing of pH in a koi pond as another aspect of the fascinating hobby called Koi Pond Keeping.

While you're at it get yourself a complete testing kit and get into the habit of testing and recording your pond test results. Always remember one koi pond test is meaningless and potentially very dangerous to your koi and therefore your own stress level.

Most of all don't stress over pond "pH problems"... almost certainly there is no pond problem unless you added "stuff" you shouldn't have done in the first place.

Articles about koi and fish pond care in winter

Pond Heaters. Heating koi ponds winter growth feeding benefits

Koi pond heating using gas electric in UK temperature control

Bringing fish and pond and water out of winter into spring

Koi goldfish pond covers over winter. How to look after pondfish

Over-wintering koi goldfish hibernation cold freezing water ice

Koi pond heating using gas electric in UK temperature control

Should I heat my new koi pond this winter. What temperature?

Temperature swings. Koi Goldfish Feeding & water temperature

To Feed or not to feed your pond fish or koi or goldfish

How Do I care For My Garden Fish Pond In Cold Winter

Frozen Koi Ponds & Fish Ponds. Ponds Freeze in Winter. Pond Care in Freezing Winter Weather

Interpet Koi Pond Test KitWhat the Does pH of Koi Pond Water Mean? Should I measure Pond pH