Pond Filters & UV Clarifiers

Pond volume decides what size filter, UV clarifier and pump you need, so it's worth getting right. Metric is easier.

Rectangular or square ponds: length × width × average depth (in metres) × 1,000 = litres.

A pond 3 m × 2 m × 0.6 m deep is 3 × 2 × 0.6 × 1,000 = 3,600 litres.

Round ponds: 3.14 × radius × radius × depth (in metres) × 1,000 = litres. The radius is half the width across.

A round pond 2 m across and 0.6 m deep is 3.14 × 1 × 1 × 0.6 × 1,000 = 1,884 litres.

Irregular or kidney shapes: work out the volume as if it were a rectangle around the pond, then take off about a third. Easier method: fill the pond with a hose and time it. If your hose fills a 10-litre bucket in 1 minute and takes 40 minutes to fill the pond, the pond holds about 400 litres.

Use the average depth, not the deepest point, unless the sides are vertical. If you have fish, round up when sizing filters: more capacity is always better than less.

Green pond water is caused by tiny free-floating algae. They thrive on sunlight and on the nutrients from fish waste, food and rotting leaves. In warm weather an algae bloom can turn a clear pond pea-green in a few days.

To fix it:

1. Install a UV clarifier. This is the only reliable way to clear existing green water fast. Water passes a UVC lamp, which kills the algae cells so they clump together and are caught by the filter.
2. Make sure the filter is sized for the pond (see what size pond filter do I need?) and running 24/7.
3. Cut the nutrient load. Feed fish less, net out leaves, remove sludge from the bottom.
4. Add plants. Water lilies and marginals shade the pond and compete with algae for nutrients.

New ponds often go green for the first few weeks while the filter settles in. That's normal and clears on its own. Browse UV clarifiers and pond vacuums.

A UV clarifier (UVC) kills free-floating algae as water passes a UVC lamp. It's the fastest way to clear green water and keep it clear.

You need one if:

• The pond is in sun for most of the day
• You have fish (which add nutrients that feed algae)
• The pond keeps going green despite good filtration

A UVC clears green water. It won't remove blanket weed or sludge, and it isn't a substitute for a filter: it works with one. Size it to the pond volume; most ponds need a UVC rated for a bit more than their actual litres. Browse UV clarifiers.

Start with your pond's volume (see how do I calculate my pond volume?). Every OASE filter and UV clarifier is rated for a maximum pond size, with a lower figure for ponds with fish and a lower figure again for koi, because fish add waste the filter has to handle. Use the figure that matches your pond.

For example, the OASE FiltoClear 19000 suits a pond of up to 19,000 litres with no fish, 10,000 litres with fish, or 5,000 litres with koi. So a 3,600-litre goldfish pond needs a filter whose "with fish" figure is at least 3,600 litres. Round up if the pond is in full sun.

See pressure vs gravity filters for picking a type, or browse filtration.

If you want the filter hidden and the pond under 15,000 litres, go pressure: we stock the full OASE FiltoClear range. If you have koi or a large pond and want a gravity filter, call us on +353 1 456 4099 to talk it through.

OASE says a UVC lamp's effectiveness drops after about 8,000 hours, roughly a year of running non-stop. The bulb still lights up, so you can't tell by looking.

Fit a new one each spring, before algae season starts, using a bulb of the same wattage and fitting as the one in your unit. We stock UVC replacement bulbs from 5 W to 55 W. Most people switch the UVC off over winter to save bulb hours: algae barely grow in cold water.

Short version: Filtral is the simplest all-in-one for small ponds. FiltoClear is the easiest to hide and the best choice for most garden ponds. A Bitron adds UVC where the filter doesn't have enough of its own, or on large ponds.

Browse OASE filtration. Send us your pond dimensions and we'll size the right one.