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Pond bacteria for green water

Updated: 22 October 2026

The pond has turned the colour of pea soup and you can no longer see the fish. Pond bacteria for green water is a narrow question: which kept product is actually sold for suspended algae, and which maker tells you to put the bottle back. Strands of blanketweed on the liner are a different problem, and this page does not become a weed catalogue. The nine listings are on the [pond bacteria comparison](/hub/garden-ponds/pond-bacteria). The Products Compared editorial team used the Royal Horticultural Society, Practical Fishkeeping, the Evolution Aqua and Envii pages, and the amazon.co.uk reviews. We did not run a clarity trial. A qualifying purchase can earn a commission at no extra cost to you. Prices are the 19 October 2026 shelf.

The pond has turned the colour of pea soup and you can no longer see the fish.

Our top picks

Evolution Aqua Evolution Aqua PURE+ Filter Start Gel, 2.5 litres

Best overall

Evolution Aqua Evolution Aqua PURE+ Filter Start Gel, 2.5 litres
9.3/10about £41.45
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Evolution Aqua Evolution Aqua Pure Pond, 500 ml

Best value

Evolution Aqua Evolution Aqua Pure Pond, 500 ml
8.3/10about £12.45
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Envii Envii Pond Klear Xtra, 1 litre

Rank 3

Envii Envii Pond Klear Xtra, 1 litre
8.2/10about £31.95
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On this page
  1. 1.What green water is, and what a UV clarifier does
  2. 2.Which kept bacteria refuse this job
  3. 3.The one kept liquid sold for green water
  4. 4.What this page will not call a cure

What green water is, and what a UV clarifier does

Green water is single-celled algae suspended in the pond. Blanketweed is the hair on the liner. The Royal Horticultural Society treats a UV clarifier as a way to clump those cells so the mechanical filter can catch them. It does not lift blanketweed, and it is not the film that deals with ammonia. Practical Fishkeeping says the flow through the UV has to be slow enough for contact. A bacterial dose does not imitate that machine.

Which kept bacteria refuse this job

Evolution Aqua says Pure Pond is not intended to clear green water, and that an appropriate UV-C is the recommendation. That covers the 500 ml, 1 litre and 2 litre tubs. The balls are monthly maintenance above 10 °C. The PURE+ gel carries the same refusal. Do not pour £41.45 of starter gel into a green pond and call it a clarifier. The monthly habit belongs on the pond bacteria buying guide.
Evolution Aqua Evolution Aqua PURE+ Filter Start Gel, 2.5 litres

Testsieger

Evolution Aqua Evolution Aqua PURE+ Filter Start Gel, 2.5 litres
9.3/10about £41.45
View offer

The one kept liquid sold for green water

Envii Pond Klear Xtra is the exception. It is 1 litre at £31.95, dosed at 10 ml per 1,000 litres. Up to 100,000 litres is a stock of those doses, not one pour. The star line is 4.0 from 1,989 reviews, and the texts split: some ponds clear, others stay green after a whole bottle. The product page allows a couple of weeks, often three to four, and up to six or eight if the water is very murky. Which pack to keep is on best pond bacteria.

What this page will not call a cure

None of the nine listings showed an HSE authorisation number. Pond Klear Xtra is sold as bacteria that bind particles, not as an approved algaecide. A wildlife pond is the barley-straw case: about 50 g per square metre, removed when it blackens. If the fish are gasping, this page stops. The filter-start logic is on pond filter start bacteria.

Common questions

Will pond bacteria clear green water?
Only one product in this set is sold for that job, and even that one does not clear every pond in the reviews. Pure Pond's maker says the balls are not intended to clear green water. The Royal Horticultural Society route for suspended algae is a UV clarifier plus a filter that can catch the clumped cells. A bacterial dose is an extra at most, not the machine that does the clumping.
Why does Evolution Aqua say Pure Pond will not clear it?
Because the product is monthly maintenance of biological balance, above 10 °C, not an emergency clarifier. The product page says so, and then recommends an appropriate UV-C. The gel page says the same about green water. UK reviews that report pea soup still green are consistent with that limit. Buy the balls when the water is already acceptable and you want a monthly habit.
Is Envii Pond Klear Xtra the kept product sold for green water?
Yes. It is the 1 litre liquid at £31.95, 10 ml per 1,000 litres, 4.0 from 1,989 reviews. It is not a lab winner, and it is not a new-filter nitrifier. The title's 100,000 litres is many doses. Some reviews clear, some do not, and the dosing chamber is hard to use. Read it as the only on-job liquid here, with that limit attached.
How is a UV clarifier different from a bacterial dose?
A UV clarifier clumps suspended algal cells so a mechanical filter can remove them. It needs enough contact time, which means the pump must not race the water through the unit. It does not replace the biological film that handles ammonia, and it does not peel blanketweed off the liner. A bacterial dose does not do the UV's job if the unit is missing or the flow is wrong.
How long should I wait before I decide the liquid has failed?
Use the maker's spread as a window, not a deadline: a couple of weeks for a first change, often three to four weeks, and up to six or eight weeks if the pond is very murky. Do not treat a ten-day brand demonstration as your guarantee. If nothing has moved and the reviews' failure stories match what you see, check the dose, the filter and the UV before you buy a second bottle on hope alone.
Is this an HSE-approved algae killer?
No authorisation number appeared on the nine kept listings. Products sold to kill algae in an ornamental pond need approval. Pond Klear Xtra is sold as bacteria. Calling it an approved algaecide would invent a status the label does not show. If a different product claims to kill algae, read the label for the authorisation before you use it.

Related pages

How to dose pond bacteria

Pond bacteria

Best pond bacteria for filter, care or green water

Pond bacteria buying guide by job, form and litres

Pond filter start bacteria for a new or cleaned filter

Sources

Public UK and maker sources from the research digest, used for this page in October 2026. Prices, stars and review counts are from the nine amazon.co.uk listings captured on 19 October 2026. A fish-safe line is a listing claim. None of these products is presented here as an HSE-approved algaecide.

  1. HSE, using biocides (retrieved 19 October 2026)
  2. HSE, COPR product types (retrieved 19 October 2026)
  3. RHS, pond filters (retrieved 19 October 2026)
  4. RHS, algae and blanketweed (retrieved 19 October 2026)
  5. Evolution Aqua, Pure Pond (retrieved 19 October 2026)
  6. Evolution Aqua, how often to dose Pure Pond (retrieved 19 October 2026)
  7. NT Labs Pond Mature (retrieved 19 October 2026)
  8. Practical Fishkeeping, green water (retrieved 19 October 2026)