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Pond filter start bacteria

Updated: 22 October 2026

The water in a new pond can look clean on a Saturday and still be a bad place for fish on Sunday. Pond filter start bacteria are about that gap: ammonia from the first waste, then nitrite, while the biological film is still thin. Colour is not the test. The nine listings and their prices are on the [pond bacteria comparison](/hub/garden-ponds/pond-bacteria). This page is only the start-up case. The Products Compared editorial team used the Royal Horticultural Society, NT Labs' new-pond notes, and the amazon.co.uk files. We did not cycle a filter in a lab. A qualifying purchase can earn a commission at no extra cost to you. Figures are the shelf on 19 October 2026.

The water in a new pond can look clean on a Saturday and still be a bad place for fish on Sunday.

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
ARKA ARKA Nite-Out II, 5 litres

Rank 3

ARKA ARKA Nite-Out II, 5 litres
8.5/10about £62.37
View offer
On this page
  1. 1.Why a new filter is a water test, not a colour check
  2. 2.Which kept bacteria are for the start
  3. 3.How to add fish while the film is still thin
  4. 4.If ammonia or nitrite will not fall

Why a new filter is a water test, not a colour check

The Royal Horticultural Society describes biological filtration as the slow part. Mechanical filtration can catch debris at once. The bacterial population takes about six weeks to build, and it can die if the filter is off for 24 hours or more. Ammonia rises, then nitrite, then nitrate. Clear water can sit in the middle of it. NT Labs asks you to test weekly for the first eight weeks, with ammonia and nitrite ideally at zero. Do not add a full stock of koi on the day the water looks clear.

Which kept bacteria are for the start

NT Labs Pond Mature, £20.39, is titled for 18,200 litres and goes into the filter. A double start dose covers about 9,100 litres from the same bottle. PURE+ Filter Start Gel, £41.45, is poured on the media and stated for 25,000 litres. The maker says the gel will not deal with green water. Nite-Out II, £62.37, is the large-pond liquid before fish go in. Leave Pure Pond for monthly care. Exact millilitres are on how to dose pond bacteria.
NT Labs NT Labs Pond Mature, 1 litre

Testsieger

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

How to add fish while the film is still thin

Add a few fish, not the full stock, and keep feeding low while the weekly tests are the main event. Rinse new foams in pond water rather than under the tap. A 10,000 litre pond with eight koi on day one is the example that goes wrong. Who should buy which starter is on best pond bacteria.

If ammonia or nitrite will not fall

Stop feeding and keep the air on. A partial water change is not a licence to empty the pond or to rinse the biological media in the sink. Call a specialist when fish are gasping, or when ammonia stays high after you have stopped feeding. If the water is green and the tests are quiet, that decision is pond bacteria for green water.

Common questions

How long until a new pond filter can cope with fish waste?
The Royal Horticultural Society figure is about six weeks for the bacterial population, and it can be lost if the filter stops for a day or more. Some keepers allow longer for the whole pond than for the media alone. A bottle can seed the media. It does not compress six weeks into a weekend. Test weekly for the first eight weeks, as NT Labs suggests, and add fish slowly rather than on a calendar promise from a label.
What should the test kit show before I add more fish?
NT Labs' new-pond note treats ideal ammonia and nitrite as zero, with pH about 7 to 8. You want those readings to stay down under a real, modest waste load, not after a day with no fish in the water. If ammonia or nitrite climbs again when you add two more fish, stop and retest before you add the rest. The kit is the gate, not the colour of the pond.
Do I rinse new foams under the tap?
Rinse them in pond water. A hard rinse under the tap is a good way to strip the film you are paying a bottle to start. The same applies when you clean foams later in the year. Mechanical sponges that are only catching silt are a different job from the biological media. If you are unsure which is which, clean less, not more.
What is the start dose of NT Labs Mature versus the 2.5 litre gel?
Mature is 10 ml per 182 litres on the maker's calculator, doubled for a new filter, a new pond, or the start of the season, into the filter. A double dose from the 1 litre bottle covers roughly 9,100 litres. PURE+ gel is one 2.5 litre bottle stated for 25,000 litres, on the media, with a listing line that you cannot overdose. Do not copy the gel's whole-bottle habit onto Mature, and do not copy Mature's millilitres onto the gel.
When is ARKA Nite-Out II the start-up bottle?
When the pond is large, new, restarted in spring, or just cleaned, and fish are not in yet. You need 7 °C or warmer, KH of at least 6 °dH, and pH 7.2–8.4, and you follow the millilitre table. It is too much bottle, at £62.37 and eight reviews, for a small goldfish pond that Mature's title already covers. The UV, skimmer and ozone stay off for about 72 hours after each dose.

Related pages

Pond bacteria buying guide

Pond bacteria

Best pond bacteria for filter, care or green water

Pond bacteria for green water, and which bottles to skip

How to dose pond bacteria from the label, not a guess

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)