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How to dose pond bacteria

Updated: 22 October 2026

The bottle is open and the UV clarifier is still humming, because last time you left it off for three days and you are about to do that again. How to dose pond bacteria starts with the label in your hand, not with a rule borrowed from the next brand. Litres, place, temperature and that product's UV interval are four different lines. The nine listings are on the [pond bacteria comparison](/hub/garden-ponds/pond-bacteria). The Products Compared editorial team lined up those amazon.co.uk files with the makers' dose pages. A qualifying purchase can earn a commission at no extra cost to you.

The bottle is open and the UV clarifier is still humming, because last time you left it off for three days and you are about to do that again.

Our top picks

NT Labs NT Labs Pond Mature, 1 litre

Best overall

NT Labs NT Labs Pond Mature, 1 litre
9.4/10about £20.39
View offer
Evolution Aqua Evolution Aqua PURE+ Filter Start Gel, 2.5 litres

Rank 2

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

Best value

Evolution Aqua Evolution Aqua Pure Pond, 500 ml
8.3/10about £12.45
View offer
On this page
  1. 1.Measure the litres you will actually dose
  2. 2.Where each kept product goes, and how much
  3. 3.Temperature and UV, one brand at a time
  4. 4.What not to combine, and when to stop

Measure the litres you will actually dose

Measure in centimetres. NT Labs' formula is length times width times average depth, divided by 1,000, for a rectangle. Blagdon multiplies an irregular pond by 0.8. A 3 metre by 2 metre pond at 0.6 metre average depth is 3,600 litres before you allow for rocks. Mature and Pond Klear Xtra need that number. Pure Pond and the PURE+ gel are sized by the whole tub. If you have not chosen a job yet, start with the pond bacteria buying guide.

Where each kept product goes, and how much

NT Labs Mature goes into the filter at 10 ml per 182 litres. Double that for a new filter or the start of the season. That you-cannot-overdose line is Mature's, not a family rule. Pure Pond is balls sized by the tub, monthly in the biological chamber, and the amazon files print no amount. PURE+ gel is stated for 25,000 litres, on the media. ARKA Pond Gel has no millilitres-per-litre figure. ARKA Special Blend, for a pond up to 1,000 litres, is 240 ml to start, then 60 ml on days 8, 15, 22 and 29. Nite-Out II starts at 30 ml up to 2,000 litres. Pond Klear Xtra is 10 ml per 1,000 litres, and the product page says every 7 days.
NT Labs NT Labs Pond Mature, 1 litre

Testsieger

NT Labs NT Labs Pond Mature, 1 litre
9.4/10about £20.39
View offer

Temperature and UV, one brand at a time

ARKA listings that print a temperature start at 7 °C. Nite-Out II wants the UV, skimmer and ozone off for about 72 hours. Special Blend wants at least 24 hours. The PURE+ maker page says 12 hours, which the amazon row did not print. Pure Pond's maker wants the balls above 10 °C, and those amazon files print no UV line. Envii's product steps for Pond Klear Xtra do not mention UV. Do not copy 72 hours onto Mature. If the crisis is ammonia, go to pond filter start bacteria.

What not to combine, and when to stop

Do not rinse biological media under the tap on the day you dose. ARKA Special Blend wants 72 hours after pesticides or algicides. Safe for fish on a bullet is a seller claim. If fish are gasping, stop and speak to a specialist. Green water is on pond bacteria for green water.

Common questions

How do I turn pond measurements into litres before I dose?
For a rectangle, length times width times average depth in centimetres, divided by 1,000. For an irregular or oval pond, Blagdon multiplies by 0.8. For a circle, 3.14 times the radius squared times the depth. Convert UK gallons by multiplying by 4.546, then allow for rocks and baskets.
Where does each kept product actually go?
ARKA gel and PURE+ gel go on the filter media. NT Labs Mature goes into the filter as a liquid dose. Pure Pond balls go in the biological chamber, ideally in a bag. Nite-Out II is dosed to the pond to build that biology. Pond Klear Xtra is mixed into a bucket of pond water and spread. Special Blend's listing does not say filter versus pond, so follow the label on the bottle you have, not a guess from a different ARKA product.
What temperature does this bottle need?
ARKA gel, Special Blend and Nite-Out II print a start at 7 °C. Nite-Out II also wants the KH and pH window. Pure Pond's maker wants the balls above 10 °C. Mature, Pond Klear Xtra and the Pure Pond amazon files do not print a degree line. Do not put 7 °C on a bottle that never stated it, and do not dose a monthly ball into water that is still at winter temperatures because an ARKA label said 7 °C.
How long do I leave the UV off, and for which brand?
PURE+ gel: 12 hours on the maker's gel page, which the amazon row did not print. ARKA Special Blend: at least 24 hours, and 72 hours after pesticides or algicides. ARKA Nite-Out II: about 72 hours for the UV, the skimmer and ozone. Envii: the product steps are silent; a brand blog says two to three days on one test. Pure Pond balls: a stockist FAQ says they do not need the UV off, and that line was not on the kept amazon files. Mature: no interval printed.
How much NT Labs Mature is a double start dose?
The standard rate is 10 ml per 182 litres. Double it for a new filter, a new pond, or the start of the season. The 1 litre bottle at that double rate covers about 9,100 litres, against an 18,200 litre title at the single rate. Weekly doses after that are for a spike.
How do I dose Pure Pond if the tub does not give millilitres?
You do not convert it into millilitres from another brand. Pick the tub whose maker band covers your litres, and add it monthly above 10 °C in the biological chamber, ideally in a bag. The amazon listings in this set leave the amount off the page.

Related pages

Best pond bacteria

Pond bacteria

Pond bacteria buying guide by job, form and litres

Pond filter start bacteria for a new or cleaned filter

Pond bacteria for green water, and which bottles to skip

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)