How to combat green water

Green water is caused by the growth of large numbers of single-celled algae, which live suspended in the water and is fuelled by light and excess nutrients in the water – typically making its appearance as the days begin to get sunnier in the spring.

Although it is most commonly encountered in recently constructed ponds, where the water chemistry and planting has not yet settled down, older, more established water features are not immune.

The principal culprit is nitrate, a nutrient which forms naturally as left-over fish food, fish faeces and dead plant material decays. Nitrate test kits are widely available and very simple to use, but often simply clearing out fallen leaves, cutting and discarding dead water-plant foliage and avoiding over-feeding your fish can make a big difference.

Natural treatment

A variety of methods can help to deal with green water. Natural methods include avoiding fertilising the pond itself – nor the area around it – removing decaying matter, not over-stocking with fish and sensible planting.

Surface plants compete with algae for light and so having as much as a half or two-thirds of your pond covered with vegetation can make a big difference, while trees or bushes can also be used to give shade, further helping to reduce the levels of sunlight reaching the water. However, these approaches may not always be enough in themselves, as green water may still happen in early spring, before the protective plants have developed new foliage.

For another natural additive, barley straw could fit the bill. Administered either as pouches of straw, or doses of straw extract, it is an approach which some have hailed as a miracle cure – though not all pond-keepers are unanimous in this opinion. It takes a month or two to show any effect, but the natural enzymes are said to have remarkable abilities to inhibit algal growth – making it effective against blanket weed also.

Chemical/mechanical treatment

Chemical treatments are also available to deal with the problem. Although they can be expensive and do not provide lasting protection, they can be very effective, particularly for newly constructed ponds. They must be chosen carefully if the pond has already been planted or stocked with fish and if you do decide to go down this route, then reading the label carefully and using the product in accordance with the instruction is obviously critical.

Perhaps the single greatest advance in recent years in the battle against green water has been the development of reliable and affordable ultra-violet (UV) clarifiers. Aside of its effectiveness in destroying the algae which cause the problem, this approach has a number of other advantages. It has no effect on water quality and because it takes place away from the pond itself, it poses no threat to the plants or fish. Easy to install and cheap to run, a good quality UV unit combined with a biological filter system and an appropriate planting regime offers the best possible means of dealing with the green water scourge – though it is important to remember to change the UV bulbs every year. A number of manufacturers produce UV units to suit a range of pond sizes, widely available from garden centres and other retail outlets.

Left unchecked, green water algae can turn the most beautiful of water features into a pitiful sight in a very short time, but the good news is that with a little careful thought and a bit of practical effort, it should be fairly straightforward to keep your pond crystal clear.

Maintaining good water quality in your pond/aquarium

The quality of your pond water will determine the health of your fish, plants and overall health of your pond. Fish typically become ill when the water quality is not correct; for example a higher than normal ammonia level; nitrite level; a pH reading that is out of the normal range will stress your fish thus making them much more susceptible to disease. With fish illness, prevention is much easier than treatment.

It is well worth your effort to try and maintain a healthy environment for your fish. As fish live in a relatively small environment any disease soon gets passed around to some or even all of the other fish. A common mistake is to think that clear water is healthy water.

The simplest way to combat poor water quality for a newly built pond is to unsure your filtration system is adequate for the amount and type of fish you intend to stock. Filtration aids are readily available which will help to kick start the biological efficiencies of the filtration system, enabling a colony of bacteria to develop, ready to cope with the first waive of fish.

Start with small quantities of fish at least one - two weeks after filtration aid was introduced. Feeding 2 pellets/sticks per fish every other day. Continue with this for the next two weeks before the next introduction of fish and continue to feed as the first stock of fish. Continue at this rate until you have reached at least half of your stocking levels testing for Ammonia, Nitrite, Nitrate & PH levels in between. Bare in mind that depending on the size of your pond, fish stocks & amount of food fed, it may take up 6month to a year for the biological filtration system to mature properly so be patient!

How to line a pond using pond liner

Firstly, you would need to work out the size of liner. This is the easiest way...

In feet, work out the length, width and depth of the hole. For example, 12' x 8' x 3'. Please note that it is important to compensate for and overlap of liner around the pond edges. We will work this out using most common overlap length, which is 2', this is how we work it out:

Double the depth (3') equals 6, then add 2' (which is for the 2' overlap) totals 8'. We the add 8' to the length (12') and also the width (8') which combined will be 20' x 16'. Therefore the total length butyl liner will be 20' x 16'. Please note that you will always have excess liner using this method but this helps to compensate for natural curves, shelves etc in a pond.

Before you line using butyl, we recommend that you add an underlay of soft sand then a non rot underlay material which can be purchased from any reputable aquatic retail outlet.

Place the butyl line into middle the pond spreading outline over the natural curves and shelves of the pond as best as possible. Make necessary folds until you have formed the outline of the pond. Then start to fill up will water. This is the easiest way as the weight of the water will flatten out any unwanted creases. As water fill ups the pond, pull the liner to even out creases and flatten folds until you have reached the desirable shape. Then cut off excess liner.

How to install a pre-formed pond

Dig a hole to fit only the bottom part of the shell up to the plant ledges because it is impossible to get and keep a preformed pond level. Place the shell in the hole. Fill the pond with water. This is important to do now, not later. The water will get dirty as you continue the installation, but you will pump it out later and put fresh water in.

Get in the water carefully. Do not stop on the ledges because they break easily and cannot be repaired. Have a helper pour the sand around the edges of the shell so it fills up the spaces where the pond does not fit perfectly in the hole. Use the water from the hose to pack the sand in. Do not get out of the water or the shell will float up and you will have to start all over. When you are satisfied the pond will stay in place, you may get out of the water. Be careful not to step on the ledges.

Make sure your pond is now sitting in the ground up to the plant ledges. Using your bricks, put them under the lip of the pond to help keep it sturdy. Place the bricks about 4" apart and go all the way around the pond. This gives some structural strength to the pond shell. Cover the bricks and spaces between to the top of the lip of the pond with topsoil. Water it in so it packs well.

Cantilever the flat rocks around your pond with about 2 or 3 inches sticking over the edge. This camouflages the lip of the pond so it cannot be seen. By now the water is full of sand and soil, so pump or wet vac it out and replace it with fresh clean water. Put de-chlorinator in the water now.

Add a waterfall or fountain, plants and fish. Plant or mulch the outside for a finished look.

How to clean a pond

For a typical pond clean start off by pumping pond water into a suitable size holding tank and ensure there is adequate aeration. Filtration systems will only need to be added if you aim to hold you fish for prolong periods and refrain from feeding fish at least 5 days prior to the clean (do not feed during the clean).

Start pumping waste pond water to a suitable position and while draining start to remove plants (keep moist), ornaments etc. Drain pond to a level shallow enough to enable easy netting of your fish and place fish stock in holding tank.

Use a thick mesh net to net out larger debris while pumping to prevent your pump from blocking. Continue pumping until your pump is unable to pump any excess water, debris etc. Use a pond vacuum to suck up sludge, debris and excess water. Brush down sides of pond and hose sides down simultaneously while still vacuuming. Continue until pond is spotless.

Start filling up pond and use an appropriate de-chlorinator to fresh tap water. Acclimatise fish correctly and switch on pumps/filtration systems.

Choosing the correct size filter system

It's important to realize that a pond filter is first and foremost required to purify the pond water ... fish pollute the water as part of their natural metabolism and if the impurities are not removed toxic levels of pollutants are reached. In such circumstances fish would become stressed and might even die. A filter is also useful to remove suspended solids from the water.

If your pond is small to medium and you do not intend to stock with expensive fish then there is little need to overspend on a filter with all the bells and whistles and hype. Most modern filters from the likes of Laguna, Hozelock, Heissner, Blagdon and Oase will do an excellent job. Price is not the determination of how good the filter is. Your choice will be between a gravity (box type) filter and a pressurized one.

To specify an appropriate pond filter it is important to know the volume of water in a pond to about 80% accuracy. A filter is specified in terms of what capacity it can handle when the pond is stocked normally which needs some interpretation. In general the more fish in a pond the greater the level of pollutants so a larger filter would be used.

A filter will claim to handle or purify up to 5,000 litres of pond water, IT CAN'T! It may be able to filter a 5,000 litre pond without fish, plants and sunshine. As a general rule, for stockings goldfish in sunshine add 50% to your pond water volume so your filter should be able to filter a pond for 7,500 litres as a minimum. For keeping koi in sunshine add at least 100% to your pond water volume so your filter should be able to filter for a 10,000 litre pond as a minimum. Use a UVC clarifier to combat green water.

Calculating pond or aquarium volume

Here is the equation:

In feet: Length x Width x Depth

= ?

then multiply by 6.23 = gallons, then multiply gallons x 4.54 = litres


Pond size - 13(L) x 9(w) x 3(d)

= 351 x 6.23 = 2186.73 gallons then multiply by 4.54 = 9927.74 total pond size

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14 Nelson Road
West Sussex
BN12 6EW

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