A Nice Clean Pond

I've got two large terracotta basins that provide permanent water in the garden. Both have rocks, some duckweed, irises and bulrushes, and both had tadpoles.  Otherwise though, the wildlife coming to these ponds has been a little disappointing: only a few hoverflies.  While the baby frogs continue to grow in one basin, they completely disappeared from the other - not sure what happened, whether it was predation (e.g. by blackbirds) or disease.  Anyhow as the water was looking a bit dirty I took the opportunity to change it, and it was gratifying to see an adult frog cooling off in there the very next day.  Hoping for some dragonflies, pond skater, etc. to move in next.