Controlling the growth of algae is one of the biggest challenges facing a koi pond owner, and any other water gardener.

The good news about green water caused by algae is that it will not harm your koi. In fact, the koi will feed on the tiny animals that feed on the algae, which can help to brighten their colours. Of course, the bad news is that you will miss out on those bright colours because you will barely be able to see your koi through the murky water. Being unable to see your koi can also have other, more sinister repercussions: you are not able to observe their behaviour, so you won’t know if they are acting out of sorts (an indicator that they are unwell) and you also aren’t able to see if they have sustained injuries or developed skin ailments.

Green water is a result of a proliferation (a ‘bloom’) of microscopic algae that contain chlorophyll (the green photosynthetic pigment). Ponds that have very low concentrations of ammonia and nitrite combined with very high levels of nitrate are most susceptible to algae problems. The nitrate is a powerful fertiliser and algae thrive when nitrate abounds. To reduce nitrate levels in your pond you can do small water changes on a weekly basis. Be sure to de-chlorinate the tap water that you use to replace the water you remove.

The fight against green water is ongoing and requires diligence, but the following can help tremendously:

1. When you build or install a koi pond (or water feature) ensure that it gets only partial sun because algae need sunlight to grow. Also, consider building your pond so that there is a section where you can plant lots of water plants without the koi having access to them. The plants will help to block out some sunlight in that section of the pond. (Sunlight is good for koi in that it makes their colours bold and bright, but it becomes a problem if the koi are unable to escape the harsh rays. Providing some shade reduces the risk of the koi developing skin cancer.)

2. The ultraviolet light emitted by the tube in a UV filter kills algae, so a UV filter is a must for every pond. Ensure that the size of your UV filter and pump are compatible the right combination will clear green water in a few days.

3. Ensure that you have a good filtration system and that it is functioning efficiently. It will reduce the toxic wastes in your pond and there will thus be less for the algae to feed on. The bigger the filtration system you install, the better your water clarity will be.

4. Use fish-friendly algaecides (algae-killing products). With most of the products you can alternate between a full dose and a half dose (usually once a week).

If you don’t have a UV filter and decide not to use algaecides it can take months for green water to clear in a newly established pond. Even after the initial algal bloom subsides your pond will still go green from time to time.

Different types of algae

Along with the free-floating algal cells that cause the water to go green during an algae bloom (and which are the easiest to remedy with UV and algaecides) there are two other types of algae that koi keepers may notice, namely filamentous algae and the velvet-like algae that grows on surfaces in the pond.

· Filamentous algae (also called blanket weed)

Filamentous algae take a number of forms they can be long and stringy, short and furry, or in the shape of webs or mats. Filamentous algae are found mostly near or on the surfaces that form waterfalls and shallow steams within ponds. It is in these areas that the sunlight is most intense, providing the heat and light that create the ideal conditions for algal growth. Filamentous algae are the hardest to combat and algaecides are usually most successful. These types of algae are not controlled by UV filters because the strands tend not to pass through the filter.

· Short velvet-like algae

The algae that grows on the surfaces in the pond and makes them look like they are covered in velvet is beneficial to the pond and it is necessary to ‘make peace’ with it. You can never get rid of it for long and nor should you want to. Initially it may scare you as its growth will be uneven and patchy in a new pond, however, after a few weeks it evens out and looks good. It gives the pond surfaces a natural appearance, does not cloud the water and does not interfere with viewing your koi. It uses nutrients from the water (making them unavailable for less welcome algae), generates oxygen during the day and gives the koi something to nibble on. {

Article written by Angela Beckx of Koi @ Jungle. Contact Angela on 031 2098781 or visit: Koi @ Jungle also stocks swimming pool products and equipment.

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