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You are here: myPetSmart.com > Pet Care Library > Articles > Algae Smart Pet Guide To Pond Care Maintenance

Algae: Smart Pet Guide to Pond Care & Maintenance

PetSmart John Gertenberger

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While almost all aquatic plants are good for the pond, some of the small ones are not. These small plants are called algae. Not only are they unsightly, possibly turning the pond into what looks like a container of pea soup, but they can use up available oxygen, and give off huge amounts of carbon dioxide.

Controlling Algae Growth

There are five ways to control algae growth:

  1. Water Changes
  2. Chemicals
  3. Filtration
  4. Shade Provision
  5. UV Sterilization

Water Changes

Algae blooms require nutrients like phosphates and nitrates. In order to control the algae you have to remove all or most of the nitrates and phosphoric compounds. This isn't easy but is doable by partial water changes. The added water contains no nitrates, so the algae growth is reduced. Partial water changes are excellent for fish health as well, so this is a highly recommended way to control algae.

Partial Water Changes
Drain some water from the pond before refilling; ideally pumped or siphoned from the dirtiest conditions from the bottom of the pond.

Remove no more the 20% of the pond volume at a time; larger water changes are likely to upset the biological balance of the pond.

Refill the pond with tap water. The only precaution necessary is the removal of chlorine, which might injure water life. Use a tap water conditioner, which will neutralize chlorine and similar chemicals.

Whole Water Changes
There will come a time in the life of any pond when the debris on the base of the pond builds up to a level that requires attention beyond seasonal maintenance.

If you decide to clean the pond, plan in advance to minimize the disruption to pond life. This is an ideal opportunity to add a waterfall, ornament, or stepping stones to your pond.

Spring is not a good time for whole water changes -- the water may be cold to work in; fish are weak following the winter and prone to stress; plants are not in full growth and therefore not suited to dividing and replanting.

Early and mid-summer are slightly better times for a clean-out, but young fish fry could be injured or lost.

Late summer is ideal -- most plants will have completed flowering, and the majority of fish fry should be large enough to be netted carefully. Pick a day that is not too hot so that plants do not dry out, and stop feeding the fish the day before.


Algicide chemicals are useful for controlling green water in unplanted ponds. Depending upon the strength of the chemicals used, you may need to re-dose at one to four month intervals.

In planted ponds you should use chemicals only when other means have failed. They can inhibit the growth of most aquatic plants, and in higher doses can kill plants. Most of the algicide chemicals have been designed to be safe to use with fish.


Some algae can be controlled by using filtration such as mechanical foam mats; however it is necessary to clean the mats daily.

Shade Provision

Algae need light to grow, as do all green plants. By restricting the amount of light that falls onto the pond's surface, you reduce the ability of the algae to reproduce and grow.

The two most common ways to obstruct light is adding some pond side plants on the sunny side of the pond, or by growing floating water plants so that about 50% of the surface of the pond is covered.

Ultra-Violet Sterilizer

An efficient method of algae control is by using a UV sterilizer in conjunction with a biological filter. They work by passing water close to a source of UV light, which damages the algae cells, preventing growth.

Using the UV filter requires you to bring a safe source of electricity to your pond. This is the job of a professional electrician. Please do not try to do it yourself. The resulting shocks might injure or kill you and your fish.

Use the UV filter with care, protecting the electrical connections from damp, and don't look at the burning lamp. Switch the lamp on whenever the water is hazy. The lamp's output fades with time, so you should replace it annually in the spring, at the beginning of the season. (Refer to your owner's manual for additional reference. )

John Gertenberger has been in the pet industry since his first jobs as a teenager working in pet stores in exchange for aquarium and pond supplies. He has owned fish and many other pets since he was a young child, and still maintains aquariums with rare tropicals. John has been with PetSmart since 1995, overseeing the live pets (fish, bird, reptile, small animal) part of the business.

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