9 spined sticklebacks 
 Young fish between 1 and 4 cm ( unsexed )
The 9 spined stickleback is the smallest native freshwater fish in the UK, mostly found in small overgrown ponds and is much rarer than it's 3 spined cousin, 9 spined sticklebacks live between 1 and 4 years depending on water conditions mainly temperature and availability of food and can reach a max size of 7 cm however a size of 5 cm is more common , they're  tiny sizes makes them perfect  for small wildlife ponds ,they are also easy to breed so you can create a self sustaining population in your pond.
9 spined and 3 spined sticklebacks can live together especially in larger well planted ponds, in smaller ponds or aquariums they do best without they're larger cousins.
Feeding: 🐛 
sticklebacks are carnivorous and will eat almost any small creatures they can fit in their mouth and will even eat hydra ( hydra vulgaris ) and can help control infestations of them.
Live or frozen-
Mosquito larvae
zooplankton 
Earth worms
White worm
Blood worm
Daphnia
Tubifex 
Water louse
Copepods 
All  make good  food sources ,due to there small size there is usually enough natural food in a well established pond to  sustain them .