This is from late spring this year. At that time there were abundant hydras and copepods. Hydras aggregated around a corner where it got light from window. At night if I put a flashlight by the tank copepods swarmed to the glass. Many of them carried egg sac.
Then since maybe mid summer the population suddenly declined. And now I can only see a handful of copepods swimming around at any time and so far only see 1 hydra.