Wildlife in this country's ponds and rivers is under threat from species from other countries - this week a new organisation is being launched to help save them.
The Freshwater Habitats Trust hopes to protect endangered British plants and animals and improve the water quality of of streams and ponds in general.
Some of the foreign creatures whose numbers are rising here are abandoned pets like terrapins which became popular pets after a cartoon twenty years ago.
Terrapins eat tadpoles and little fish. Others species arrive in egg and larva in clothes, cars and boats.
Another example is the British water vole which is under attack from the American mink, thought to have escaped from UK farms.
Pond Conservation told Newsround: "We're worried about alien species because they are so difficult to control. But their effect on wildlife pales into insignificance compared to the damage caused by pollution.
"It's shocking, but clean, unpolluted, water - which is vital for freshwater life - has all but gone from large parts of the country... leaving most rivers, streams and ponds much more impoverished than they should be."