One of the most important pieces of equipment for any Koi enthusiast, unless they particularly like 'Green Water' and don't wish to be able to see their prized possessions, is....
The Ultra Violet Sterilizer/Clarifier!
The question is what is this algae that causes pond keepers a problematic struggle each year?
Algae is hundreds and thousands of microscopic single-celled plants individually called alga. When we usually see this, it is in the form of a green mass caused when they are in abundance.
Where did they come from in the first place?
Single-celled green alga are a spore and the alga we are talking about here is plank-tonic algae which will bloom in good light and also in nutrient-rich water. By nutrients we mean not only the light but nitrates and phosphates which can be introduced to a pond with fish, fish food, or just about anything else you put in the water. It can be produced by over feeding, or even bird and wildlife traffic. Poor circulation and aeration also play a part. Remember algae itself by its nature depletes the pond water of much needed oxygen.
Any of these circumstances can throw off the balance of a pond's ecosystem, and algae will quickly take advantage of that imbalance. They will grow rapidly and only die back when the nutrients are depleted. Even non-toxic chemicals that you treat your lawn with can run off and reach the pond inviting an unwelcome algae explosion.
There are other things that you can do to help, by cutting down on excessive light for example, when constructing your pond plan for a shaded spot or protected it by way of a pergola or overhanging plants. The small diameter of the green water producing Chlorella vulgaris cells means they can pass through all conventional filter media and all but the finest open-celled foam, so physically filtering them out in their normal form is next to impossible unless the medium is partially blocked. However, passing the cells through a UV disrupts their internal structure, if not actual killing them, so that they clump together or 'flocculate'. These clumps, being of a relatively large diameter, can be netted out or trapped and broken down by the filter.
When to use a UV Sterilizer/Clarifier?
Most Koi keepers with the sufficient water depths that Koi require will keep their unit(s) switched on 12 months of the year as they do with their filtration, but the owners of smaller garden ponds may just find that they only need the UV on in the spring through to late summer, with the combination of longer days and an upsurge in fish activity (and, therefore waste production) is when the UV is necessary. If you intend to run your UV year-round, it must be totally protected from frost in a well-ventilated housing. Again, the recommendations vary depending on the manufacturer, some suggesting that the unit be dismantled and stored dry over winter. Please read the relevant instructions for your specific unit.
How much UV do I need for my pond?
Purely for green water eradication, 8-10 watts per 1,000 gallons is usually recommended, but some pond keepers will maintain that, for a reduction of bacterial levels, this can be increased threefold to 30 watts. However, this is an oversimplification. Pond capacity is not the only factor to take into consideration - stocking levels, how much sunlight the water receives and the amount of shading will determine which size UV is right for you. Remember this is one thing that you can't 'overdo' but it is too easy to 'under do'.
Obviously for maximum efficiency you should run your UV in conjunction with a pump rated in gallons per hour to suit the size of your pond. The longer the contact time of the water with the UV light, the more alga spores will be treated, but there is a lower limit to the throughput of water, beyond which filter efficiency and/or aeration via the Venturi may suffer. If your pond has only one UV and one run of return pipework, you may have to compromise, on the basis that the pump must circulate the entire pond volume once every two hours (as an example, a 1,000 gallon pond should be teamed with an 8-15 watt UV and pumping rate of 500 gph minimum). One way of irradiating a large pond with only one UV is to T-off some of the flow through the unit. This treated water need not necessarily be returned direct to the main body of the pond - it can be diverted back into the filter. If you have two UV's plumbed in parallel servicing the same pond you can effectively double the contact time in each UV unit which substantially increases the amount of irradiation. The latter is a useful option in very large ponds, and some manufacturers offer tandem UV units.
Bearing in mind what we have mentioned above - Using the correct size UV not only clears 'Green Water' but also enables you to be able to view your Koi in crystal clear water, and as the term 'sterilizer' infers, it also sterilizes the water itself - thus helping to kill the harmful bacteria that lives with in it and which can also cause problems to our valuable Koi - So a correct size UV is very beneficial all round..
Of course, there are many other types of Algae but these need to be tackled in different ways.