Natural Pond Handbook

45 · Pond Microbiology — The Invisible Web

Illustration of pond microbes
Tiny life on every surface does the real filtering.

Clear water is a living filter. The slime on every rock is a city of microbes doing the real work.

Biofilms

That slippery coat on liner, rocks, and plant stems is a biofilm — bacteria, algae, and protozoa in a self-made slime. It is the pond's primary digestive system, breaking down organics on contact.

Nitrifiers & heterotrophs

  • Nitrifiers (Nitrosomonas, Nitrobacter) convert toxic ammonia → nitrite → nitrate.
  • Heterotrophs consume dissolved and solid organics, multiplying fast after a feed or leaf drop.

Why sludge forms

When incoming organic matter outpaces the heterotrophs, solids settle as sludge. Anaerobic pockets in deep sludge then release methane and hydrogen sulfide — the rotten-egg smell of an unbalanced pond.

Feeding the microbes

Microbes need surface area and oxygen. More planted, textured surfaces (bio-balls, lava rock, plant stems) and steady aeration mean a faster, more stable cycle. Avoid over-cleaning — you are scrubbing away your filter.

Balance A healthy pond smells of nothing, not of earth or sulphur. Odour is the first sign the microbial web is overwhelmed.
Understanding pond microbiology

Every litre of pond water contains billions of bacteria, protists, and microscopic organisms. These are not a problem — they are the engine of your pond. They decompose waste, cycle nitrogen, and keep the water clear. Your job is to provide the right conditions for them:

  • Surface area for colonisation (gravel, filter media, plant roots).
  • Oxygen (aerators, surface agitation, oxygenating plants).
  • A steady food supply (fish waste, decaying plant matter).
  • Stable pH and temperature.