Frequently asked questions on green water


Jeremy Gay answers some of the most frequently asked questions on green water and explains how you can keep your pond water crystal clear.

What is the best way to get rid of green water?

Green water is caused by microscopic algae living in the water column. The most effective control is to use a combination of a pond pump, ultra violet clarifier (UVC) and a pond filter. Algal cells are damaged by the harmful UV light so continuous use should ensure clear water at all times.

Other methods include algicides — chemicals that kill the algae, like a herbicide — though these are merely short-term fixes that don’t address the initial cause of the algae. Chemical filter media like phosphate removers can help to prevent or starve green water by removing their food – the nutrients from the water.

Aquatic plants are nature’s algae battlers as they use excess nutrients for their growth, depriving algae of them as they go, and, by shading them, cutting down the amount of available light reaching the algae.

Do any animals eat the green water?

Yes, green algae is close to the bottom of an aquatic food chain and predated by lots of microscopic organisms which, in their turn, are eaten by fish. Daphnia can clear green water over time and numbers can be limited if insufficient algae and food are present.

However, in a filtered pond the Daphnia will quickly be eaten by the fish so cannot do their job properly. Yet it is still a great natural solution to still, fishless wildlife ponds.

I keep changing the pond water for beautifully clean tapwater, only to watch it go green again within a few days or weeks. Why is this?

Green algae cells only need light, water and nutrients to survive. Every time you change the water you are also adding nutrients from tap to pond, including nitrates and phosphates which the algae use as food.

Leave the water, which, if dechlorinated is perfectly good for fish, and the green algae will strip the nutrients until there is none, then crash.

PFK has received many enquiries from readers experiencing green water. Every expert we associate with has recommended just leaving the problem to sort itself out, unless you want the more instant and prolonged results gained from using a UVC.

I bought a pump, UV and filter package that guaranteed clear water for my pond. Six months on and it still isn’t clearing it. Why?

If the pump UVC and filter have been matched up by the manufacturer, the problem could be at your end. Have you calculated the pond volume accurately and what volume figure did you give the shop assistant when you bought it?

Check and re-check the dimensions and get an expert to calculate the volume for you. If the package is undersized for the volume of water it may never clear your problem.

If you fitted the UV bulb, is it working? Wait until dark then see if a purple light is glowing inside the filter.

Have you split off or manipulated the water flow in some way? These packages need all the rated flow to go through the filter in a cycle — not “teed” off so that some of the flow goes through an ornament. Is the pond situated in full sunlight? A badly placed, badly designed pond will be much more prone to algae problems.

What is the fish load in the pond and how much are you feeding the fish? These clear water guarantees are based on averages and ten goldfish produce far less waste than ten Koi.

Once all these factors have been checked out and eliminated, tell the manufacturer you want it sorted. You may be offered a model upgrade, discounting the price of the original system from the new larger one. Guaranteed means guaranteed, so fight your corner.

From experience I always like to choose a system that will filter at least half the pond volume added again, or even twice the volume. This will seem like a waste of money at the time but buying a small system not up to the job is false economy.

I’ve put together my own filtration system and the green water isn’t clearing. Why is this?

Nine times out of ten the pump will be too powerful for the UVC, and UVCs have a recommended or maximum flow of water to allow the algae cells enough contact time with the harmful UV rays.

If the algae whizzes too quickly past the UV bulb, there won’t be  sufficient contact and the green water will not be cleared. Use a fine mechanical filter after the UVC to catch the disrupted algal cells.

Which is better? One long UV tube or a compact one?

Again it is all about contact time. Compact UVs only work because the water is spun around the light for better contact time. For large ponds and commercial systems everyone uses long, linear tubes, so these are probably the most effective.

How often should I change my UV tube?

Six monthly is best, unless the manufacturer states otherwise. This doesn’t necessarily mean two bulb changes a year, just ensure you have a new UV bulb for the spring/summer season. Many people turn off UVs or remove them completely in winter, so a new bulb from March to September will be fine.

This item was first published in the August 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.