The odd bubble on your garden pond is probably down to a fish or passing frog. The bubble pops, the ripples fade and your pond is a tranquil mirror once more. But when your pond is disfigured by a froth of dirty white or green bubbles that cling to the surface like a permanent scum, it's time to find out exactly what's causing them and what you can do to burst those bubbles for good.
Ponds with a waterfall or fountain are bound to have some bubbles, caused by the agitation of the water. These bubbles are natural and harmless. If you spot a patch of foam and you're not sure if a water feature is the cause, turn off the pump and wait for the water to settle. If the froth persists, then the bubbles have some other origin.
Bubbly froth on your pond is typically the result of too much organic material dissolving in the water. This could be because rotting debris has built up at the bottom of the pond, or because there are just too many fish living, eating and excreting in the space. The resulting compounds, known as phosphates, indicate increased levels of ammonia and nitrates in the water, which can be hazardous to fish.
Increased phosphates may be down to over-feeding your fish. If the troublesome bubbles persist, cut back the amount of fish food you add to the water and see if this makes a difference. Where over-feeding is the cause, the bubbles should vanish in a week or so.
Phosphates in the water are caused by organic material rotting and dissolving into it, so you might think the obvious answer is to remove all the organic material you can. However, one reason for such a build-up might be that there aren't enough plants in there. Green plants use up the nitrates from fish waste. Add some more pond plants and you could see that bubbly tide recede.
Filters and foaming agents
Your pond filter may not be working well and this can result in bubbly froth forming. Lift your pond filter out and give it a clean. If this doesn't remedy the problem, try replacing it with a larger one. Alternatively, try a pond de-foaming agent, a chemical remedy available from your local garden centre.
Tap water troubles
Tap water contains dissolved organic nutrients, so if you've been topping up your pond from the tap, this can feed the froth. Remove several bucketfuls of the affected water and replace them with fresh rainwater. Do this over a period of days, to gradually change the composition of your pond. Install a rainwater butt to collect water for topping up the pond in future.
Dredge your pond with a net, to remove rotting organic material such as leaves and fish waste from the mud at the bottom. Cut back any bushes, shrubs or trees that overhang the pond, to prevent leaves dropping into it in future. If there is a lot of vegetation around the pond, put netting over it in the autumn to stop plant debris falling in.