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Pond bacteria or flocculant

Updated: 22 October 2026

You have two bottles that both claim to clear a garden pond, and the water is still green on a Saturday. One is Envii Pond Klear or Pond Klear Xtra, a weekly bacterial treatment. The other is a clumping bottle such as Tetra CrystalWater or Blagdon Green Away. Pond bacteria or flocculant is a choice of clock, because the names overlap and the jobs do not. Envii measures very murky water in weeks. Tetra and Blagdon describe particles gathering in hours. The split belongs with the [pond clear comparison](/hub/garden-ponds/pond-clear). Once you know the mechanism, [best pond clear](/hub/garden-ponds/pond-clear/sub/pond-clear-best) can name a conditional pick, and the litres belong on [how to dose pond clear](/hub/garden-ponds/pond-clear/sub/pond-clear-how-to-dose). This is not a laboratory test.

You have two bottles that both claim to clear a garden pond, and the water is still green on a Saturday. One is Envii Pond Klear or Pond Klear Xtra, a weekly bacterial treatment.

Our top picks

Tetra Tetra Pond CrystalWater, 500 ml

Best overall

Tetra Tetra Pond CrystalWater, 500 ml
6.3/10about £11.30
View offer
Envii Envii Pond Klear, 500 ml

Best value

Envii Envii Pond Klear, 500 ml
7.8/10about £12.95
View offer
Blagdon Blagdon Green Away, 250 ml

Rank 3

Blagdon Blagdon Green Away, 250 ml
8.4/10about £7.95
View offer
On this page
  1. 1.Two different clocks
  2. 2.Choose the bacterial bottle when
  3. 3.Choose the clumper when
  4. 4.When neither bottle is the tool

Two different clocks

A bacterial bottle and a flocculant are both sold as pond clear, and you cannot swap their doses. Envii Pond Klear Xtra is bacteria. The UK page states 10 ml per 1,000 litres, weekly, and it allows 6 to 8 weeks for very murky water. Pond Klear is weekly too, at 25 ml per 1,000 litres on Envii's page, while the amazon.co.uk listing does not print a millilitre rate. A flocculant gathers particles so they fall, or so a filter can catch them. That is not the same as killing algae. Tetra's UK page describes flocculation at 50 ml per 1,000 litres, with carbonate hardness of at least 6 degrees dH, and a line that it is not there to combat algae. Blagdon's FAQ says Green Away coagulates green-water algae at pH 6.5 to 8.5. The listings are shorter: binding within a few hours, or clumping suspended algae. Pouring bacteria on Saturday for a clear Sunday uses the wrong clock.

Choose the bacterial bottle when

Choose Envii when you can dose every week and you can wait. Xtra's page allows a filter or none, which does not shorten the 6 to 8 weeks for very murky water. Reviews often say the pond is still green after several weeks, which sits inside that printed clock. Pond Klear and Xtra are one mechanism at two rates. Xtra's listing dose is 10 ml per 1,000 litres. Pond Klear's listing omits millilitres, and Envii's page states 25 ml per 1,000 litres. A 4,000-litre pond is 40 ml a week on Xtra and 100 ml a week on the Pond Klear page rate, so stay on the label you bought. Bacteria fit worse if you want particles gone this afternoon. Sort pea soup from silt on the clear green pond water page before you commit to weeks of dosing.
Tetra Tetra Pond CrystalWater, 500 ml

Testsieger

Tetra Tetra Pond CrystalWater, 500 ml
6.3/10about £11.30
View offer

Choose the clumper when

Choose Tetra CrystalWater or Blagdon Green Away when you want suspended particles gathered soon and you will read the chemistry line first. Tetra's listing talks about binding within a few hours. Blagdon's talks about clumping suspended algae. That weekend clock still does not mean the flocculant kills algae. Do not merge Tetra's UK page with the reviews. The manufacturer limits CrystalWater to mineral and organic particles, says it is not to combat algae, and sets KH at least 6 degrees dH. The listing omits those limits. Suspended algae sits closer to Blagdon, inside pH 6.5 to 8.5. Silt sits closer to Tetra if the KH is high enough. Blagdon reviews include brown globules and a dosing chamber that runs back. Tetra reviews include dead fish. Dead fish means you stop, rather than pouring a second bottle into an unmeasured pond.

When neither bottle is the tool

Barley straw and a UV clarifier are the exits when neither weekly bacteria nor a same-day clump is the job. The Centre for Aquatic Plant Management's barley-straw sheet describes loose straw in still water, twice a year, about 10 g per square metre as a minimum and up to about 50 g per square metre for a dense problem. Those grams are not a bottle dose. Straw will not replace Envii's millilitres or Tetra's 50 ml. Blagdon's FAQ points to UV for ongoing clarity after Green Away has coagulated the algae. AquaDesign and the NBOFOL pouch do not state a mechanism, so they are not a third column. Use bacteria if you will dose weekly and can wait 6 to 8 weeks. Use the clumper if you need hours and the KH or pH line is met.

Common questions

Which one works this weekend?
A flocculant is the bottle with an hours clock. Tetra's listing describes particles binding within a few hours, and Blagdon describes clumping of suspended algae. Envii is the wrong answer for a weekend, because the Xtra page allows 6 to 8 weeks for very murky water. A clump can leave the pond brown first, and Tetra's UK page says CrystalWater is not to combat algae.
Which one is the weekly habit?
Envii Pond Klear and Pond Klear Xtra are the weekly products in this set. Xtra's stated rate is 10 ml per 1,000 litres. Pond Klear's page states 25 ml per 1,000 litres, even though that listing does not print the rate. A flocculant treats a particle problem. It is not a standing weekly culture.
Why does Tetra's site say not for algae when reviews mention green water?
Tetra's UK page limits CrystalWater to mineral and organic particles and says it is not to combat algae. The amazon.co.uk listing omits that line, and some UK reviews say green water cleared. The page is the maker's claim. The review is one pond on one day. Blagdon is the listing here that states clumping of suspended algae, and a review does not cancel a KH of at least 6 degrees dH.
What does brown or cloudy water after a dose mean?
On a flocculant, brown globules or a milky phase can be the clumps the product is built to make. Blagdon reviews mention those globules, and a dosing chamber that runs product back. Clouding in the first hours is not the same event as dead fish in Tetra reviews, which means you stop rather than add another bottle.
Do I still need a UV?
You may, if you want the clarity to last after the clumps settle. Blagdon's FAQ treats UV as the follow-on once Green Away has coagulated green-water algae, especially where large light particles need a filter. A UV does not choose between the two bottles. It matters when light and nutrients put the green straight back.
Is barley straw either of these bottles?
No. Barley straw is a loose, twice-a-year control from the Centre for Aquatic Plant Management, not a bacterial liquid and not a flocculant. The sheet's figures are grams of straw per square metre, which you should not turn into millilitres of Envii, Tetra, or Blagdon. Straw will not clear a pond this afternoon, and it will not replace a weekly bacterial dose.

Related pages

Pond clear buying guide

Pond clear

Best pond clear for green water, with one condition

Clear green pond water when the pond looks like pea soup

How to dose pond clear from the litres you measured

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 six amazon.co.uk listings in this set, shelf date 19 October 2026. A fish-safety line is a listing claim, not an HSE approval. A 4.2-star row is not evidence that fish were unharmed.

  1. RHS, pond algae and blanket weed (retrieved 19 October 2026)
  2. Froglife, pond problems (retrieved 19 October 2026)
  3. HSE, COPR product types (retrieved 19 October 2026)
  4. Tetra UK, CrystalWater (retrieved 19 October 2026)
  5. Blagdon treatments FAQ (retrieved 19 October 2026)
  6. Envii Pond Klear Xtra (retrieved 19 October 2026)
  7. GOV.UK, great crested newts (retrieved 19 October 2026)
  8. That Pond Guy, calculating pond volume (retrieved 19 October 2026)