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Pond maintenance

Has your pond or water feature gone to pot? Then read our complete guide to cleaning and maintenance.

Over the past few years the popularity of garden landscaping and design has encouraged many of us to install water features in our gardens. They look fabulous initially, but over time they can turn into slimy, mosquito-infested bogs. Green water algae (or pea soup algae as it's also called), together with slime algae and blanketweed, are some of the many frustrations of pond and water feature owners. 

algae ALGAE

Algae are like the aquatic version of garden weeds, if you like. And like other weeds, they need light and nutrients to survive. If the surface of the water is constantly exposed to sunlight, algae will thrive. And if there's an abundance of nutrients in the water and not enough plants to compete for those nutrients, algae will be thanking you for that as well.


  • The simplest way to discourage green water algae is to shade the surface of your pond with floating plants and water lilies. The more plants you have the better. As a rule of thumb, you should be aiming for a leaf coverage of about 75%.
  • Oxygenating or submerged plants will also help to keep down the algae. Oxygenators are leafy, soft plants with small roots; they depend on their leaves for the absorption of nutrients, which are also required by algae. In the competition for nutrients, the oxygenators generally win and gradually starve out the algae. One of the best oxygenators to use is Hygrophylla polysperma . Another very good one is Hygrophylla diformis , which is actually used overseas in sewage water purification plants. 
  • There are algaecides on the market that will clear your water (such as Algae Fix). These are OK to use with fish, so long as you use the correct dosage. (Overdose, and your fish could die.)
  • For organic control, Bioactive Algaway and Pond Zyme are a couple of good products to use. Bioactive Algaway contains a natural plant agent that helps break down the cell walls of algae. Pond Zyme consists of either bottled or powdered bacteria which consume the organic wastes and nutrients in the water, thereby starving out the algae. Both of these, however, must be used on a regular basis - that is, fortnightly. Whereas chemical algaecides will kill algae in three days, these organic products will take six to eight weeks before you start getting a result.
  • UV Clarifiers. If you want to absolutely guarantee clear water, use a UV clarifier. What happens is, water passes through a sleeve which contains a UV bulb that kills off the algae.

blanketweed algae BLANKETWEED

The blanketweed algae is a lot more persistent and difficult to control. It's a filamentous algae that looks like green cottonwool and its spores are spread via the wind. Blanketweed is a particular problem with concrete water features and water features that have river boulders in them, because it likes a high pH, and those things create a high pH.


  • To control blanketweed you need to manage the pH, maintaining it around neutral (between 6.5 and 7.5). There are products available to do that, such as Pond Balance.
  • There are also algaecides that will kill it, but that's not going to solve your problem long-term. It will give you short-term relief, but if you haven't changed the water conditions, it will just start regrowing again once the algaecide wears off.
  • Partial water changes is something else pond/water feature keepers can do regularly throughout the year, scooping up any blanketweed or dead leaves and plant debris as you go. About 30% of your pond's water should be removed every six to 10 weeks. Simply grab a bucket and drain, then top up with the hose. Or here's a great idea — why not dunk the watering can into your pond every now and then and use that water on your garden. Then, every now and then, just top up your pond.


mosquito larvae Fish are ideal for water features because they're the best means of controlling bugs and pests (such as mosquito larvae and water lily aphids). Every now and then you just need to give your plants a blast with the hose so the bugs drop into the water for the fish to eat. If you notice mosquito larvae in your pond, stop feeding your fish for a while. Hungry fish make the best mosquito predators.

By the way, mosquitoes hate moving water, so if you keep the water moving (with a fountain or waterfall), you won't have a problem. What they do like is shallow, stagnant water. So the steeper the pond's sides, the less sheltered area available for mosquito larvae to live. The egg-laying females will seek out the sheltered spots — even in the top of planter pots that are half submerged in water, or the shallow water in between rocks. Both these places provide an area for mosquito larvae to escape from fish.

If you don't have moving water, here's a clever trick to discourage mosquitoes — add a few tablespoons of vegetable oil to the water. Mosquito larvae breathe through air tubes that link to the water's surface; the oil prevents them from obtaining oxygen by forming a barrier at the surface. But a word of caution, if you have fish, do not use this method — you shouldn't need to, anyway. The oil will inhibit gas exchange (that's the giving off of CO2 and the taking in of oxygen) at the surface and as a result suffocate the fish.


fish If you've got fish, they will need some special attention coming into winter. Being cold blooded, their metabolism rate is governed by the temperature of the water. In the winter, they're slowing down. When the water temperature reaches about 10 degrees — around May — it's important to feed them a winter food. A winter fish food is lower in protein and higher in digestibles, and is usually based on wheat germ. The reason you feed them that through winter is that, with the lower temperatures, their digestive system doesn't work as well and you start to get partially digested food in the stomach. When spring comes and the weather warms up, that starts to ferment and they get health problems. When the water temperature gets down to about 5 degrees, stop feeding altogether. In Auckland, that's unlikely to happen, but in Invercargill, for example, definitely.


Fish can handle the cold weather no problem. They'll just go to the bottom of the pond and remain inactive until the weather warms up. If you're down south and your pond freezes over, simply cut a hole in the ice to allow for gas exchange. Never smash it with a hammer or the shockwaves may kill your fish.

One word of advice — for those of you in the deep south who use a UV clarifier for the control of green water algae, dismantle it and bring it inside for the winter. If the water that's inside them freezes, it can rupture the glass sleeve that's inside.


You may have noticed that many ponds experience a sudden flush of algae in early spring. The problem is, your plants, especially water lilies, will either die down in winter or become dormant. That in itself isn't so bad, because the algae will also die down or become dormant as light conditions drop. But come spring, the algae will often get a head start, growing a lot faster than the plants. That's where a winter clean-up is essential.

If there's a lot of dead leaves and fish droppings sitting at the bottom of your pond, rotting and releasing nutrients into the water over winter, the algae is going to feed on those nutrients come spring and get a good head start. Over winter clean out any residual blanketweed, leaves or plant debris to reduce the nutrients in your pond.

Keep an eye on the pH, too. As mentioned, the ideal pH for pond water is between 6.5 and 7.5.


There's one last thing to consider if you own a water feature. Once a year you should undertake a complete clean-out. June, July, August is the perfect time for that because most plants will be dormant, and you'll be heading into spring with a clean pond.

Take the whole lot out. Drain it, clean it, remove all the sludge and debris from the bottom, and divide your plants. Hardy water lilies can be placed back into the clean water, but tropical water lilies should be lifted and stored in moist sand over winter.

Good old elbow grease is all you need. Never use any detergents. A really good scrub with a stiff brush is enough for most people, but if you've got a really bad case of sludge, you can waterblast it.

Winter's also a good time to spray neighbouring fruit trees for aquatic pests. While water lily aphids disappear from your pond during winter, they will overwinter in nearby fruit trees, especially cherries and plums. Spray with a winter oil to reduce these little nasties next season.

Aponogeton distachyos And if you've done all that and want to introduce some colour into your pond over winter, consider the water hawthorne (Aponogeton distachyos). This plant has oval leaves and vanilla-scented flowers that bloom well into winter. Feed fortnightly with a water lily fertiliser. Apart from its favourable winter-flowering appeal, it will grow in deep water and in moving water, where water lilies will not.

Thanks to Jansen's Pet & Aquatic Centre.


All products mentioned in above article can be purchased at Jansen's Pet & Aquatic Centre.

Reproduced with permission from NZOOM Home and Garden content,
from the previous website of  TVNZ News

The views expressed here are not necessarily those of the RNZIH

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Last updated: June 2, 2004