With so much chemistry going on in your pond, having a look through a pond algae troubleshooter might just save you the heartache of a green and stagnant pond.

We all know the type of pond algae problem we are talking about, the stubborn ‘pea soup’ pond, where a single or series of factors may have led to algae gaining a rapid and strong hold on your pond, blocking your view of the fish and even endangering their lives.

Every year, especially during the Summer months, we receive hundreds of calls at Swell UK asking for algae control troubleshooting help, so here it all is for free!

What causes algae?

That greenish tinge to your pond, or layers of thick blanket and string algae is basically aquatic vegetation – photosynthesis gone crazy! Whether it’s made up of single celled algae, free-floating in your pond, or the stuff that bonds together to form a soup, it’s always powered by the sun and the nitrogen cycle within your pond.

The main causes are an abundance of sunlight on the algae, oxygen levels and excess ammonia in your pond water. But the good news is that there is an even longer list of things you can do to combat it.

Using pond test kits and labs

A fast track way to sorting out your algae problem is to identify major factors contributing to it. Using one of the many reliable pond test kits on the market is the best way to do this. Most give you instant and precise results about pH levels, ammonia levels, nitrite and nitrate quantities and much, much more. One of this kits should be your first port of call to identifying where the problem stems from.

Algae filters

Pond algae problems often develop in abundance due to your pond filter’s inability to deal with the algae. This may because the pond is relatively new (take a look at our articles on New Pond Syndrome), or because the filter doesn’t have the right capacity to handle the water (or rather, the waste) in your pond.

In a perfect world, ammonia (which in this case is basically algae fertiliser) is captured by your filter media and broken down by the colonies of nitrifying bacteria that make a home of your filter box. But if your pond is too heavily stocked with fish and therefore producing more waste, or your filter is simply not up to the job in terms of size of pond (water in the pond should be circulated every 1 to 2 hours for optimum cleaning), then it might be time to consider selling a few fish, or splashing out on a new filter.

It could be that your filter needs time to mature (develop enough bacteria). Consider using a bacteria booster treatment (premade cultures of bacteria) to help things along.

Pump and filter clean

Make sure your pump is clear of debris – an impeded impellor impedes your whole filtration system from doing it’s job and clearing algae and ammonia. You can also manually clear your filter sponges from larger bits of debris and wash them in dirty pond water can help too (don’t use clean tap water as this may kill off the nitrifying bacteria in your pond).

Manual algae clearing

Not often the most pleasant of jobs, and not effective for single celled algae (green water), but blanket weed and string algae can be cleared to some degree with a pond net. However if you don’t treat the underlying factors in your water chemistry that caused the algae in the first place, you are likely to see it return swiftly!

Sludge and debris

It could be that you have a nasty layer of sludge decomposing at the bottom of your pond. This is usually caused by a mixture of fish waste, uneaten food and decaying plant matter. During the Autumn and Winter, leaves can land on your pond, become waterlogged and sink, leaving them to decay at the bottom and releasing huge volumes of extra ammonia into your water, just ready for the summer sun and to feed a big algae bloom.

Use a pond sludge buster, or pond vacuum to clean your water of debris, and consider a pond net to keep out leaves over the winter. Using a pond skimmer to constantly clear the surface can be a great way to keep things fresh too!

Garden Fertiliser

Check that your use of garden fertiliser or plant food elsewhere in the garden is not leaching through to your pond, via either soil leaching (not too likely if you are using a pond liner), or through surface run off. Check out the labels on your fertilisers too, ensuring that this isn’t a factor. Many contain ammonia which not only helps your plants grow, but your algae too.


As previously touched upon, keeping too many fish in a pond can cause a lot of fish waste, packed with ammonia. Talk to Swell or your local fish stockist about your pond volume and how many fish are in it. Don’t forget to include the type and size of fish too, as Koi tend to produce about 4 times a much waste as goldfish.

Excess food wastage

Review both the quantity of food you are feeding your fish, the type of food, and how you feed.

Perfect pond fish feeding gives your fish a meal that is best suited to them, ensuring it gets eaten (not great to give surface feeders sinking pellets, and bottom feeders surface flakes). Try feeding your fish as much as they can eat within 2-3 minutes, using a fine pond net to remove any uneaten food, which would otherwise sink to the bottom and add to ammonia levels as it rots.

Algae Treatments

Fortunately, the pond supplies market is packed full of algae treatments, helping to breakdown both free floating algae particles and string/blanket algae. Featuring a range of both biological and chemical algaecide methods, you may be stuck for choice – at Swell UK, we find Cloverleaf Blanket Answer flies off our shelves faster than most, as well as barley straw extract and bales which can help too.

Water change

Consider more regular water changes using dechlorinated water, ensuring you use a pond test kit throughout to make sure other factors do not fluctuate too violently, upsetting any balance you may have achieved.

Sun exposure

A little more difficult to deal with is the issue of sun exposure. If you pond gets a lot of sun all year round, it arguably might be in the wrong place. Giving the sun free reign over your pond means the algae particles get all the sunlight they need for photosynthesis, meaning growth and spreading is made even easier.

Consider finding a way to block your pond from the worst of the exposure – some people use hanging gardens over their pond to great effect, but beware of falling leaves in the autumn which may add to ammonia levels.

Extra help – UV clarifiers and pond plants

It may be necessary to bring in the big guns. Many pond keepers employ algae control methods as a matter of course, with inbuilt UVC’s in their filtration system.

If you don’t already have one of these electric algae killers, you might want to consider it. They work by shining a powerful Ultra violet light onto the water before it enters your filter, causing the algae particles to clump together so they can be captured by your filter media and broken down.

These handy UVCs come in different strengths and sizes, so just match them to the flow rate of your filter and pump system, and don’t forget to change the bulb every 6 months as a minimum – despite the light being on, the UV output will diminish overtime, making it a useless expenditure without fresh bulbs every 2 seasons.

Pond plants can also be used to great effect. While many require work of their own in terms of trimming and maintenance, many also feed on the ammonia your algae feeds on, providing a little natural competition, potentially starving the algae out. They can look great too if you get the right plants with the right colours to compliment your garden.

The final word on algae

Few pond keepers could keep a straight face if they ever spoke the words “I’ve never suffered from algae problems”. For most, especially in the early stages of a pond, it’s inevitable, and as this blog has demonstrated, there are plenty of factors involved.

The last piece of advice we can give you is patience. Seasons change, filters mature and fish stocks fluctuate, meaning algae problems come and go with the right kit, care and advice.