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Individuals of the Paedocypris genus can be just 7.9mm long at maturity, scientists write in a journal published by the UK's Royal Society.
But they warn long-term prospects for the fish are poor, because of rapid destruction of Indonesian peat swamps.
The fish have to survive in extreme habitats - pools of acid water in a tropical forest swamp.
Food is scarce but the Paedocypris - smaller than other fish by a few tenths of a millimetre - can sustain their small bodies grazing on plankton near the bottom of the water.
More.