Question
My pond is about 2m x 2m with a max depth of about 1m. About 75% plant cover. About 14 goldfish ranging from 3" to 6" + frogs.

Until about 3 weeks ago, the fish would gather near the surface around mid-day for feeding. Then they all dissappeared! We have been visited by herons which I've seen in early mornings so I thought the fish had all become heron takeaways.Then about a week ago I spotted a grass snake in the water. These can grow to about 1m long. Then I noticed all the frogs had gone. So I'm now thinking that the snake got the frogs and the fish. Now the snake seems to have gone and, as if by magic, most of the fish have reappeared!
My hypothesis is that the fish got traumatised in some way by an attack of some sort and as a defence mechanism, crash dived and stayed out of sight on the bottom. If true, this suggests that the fish might have some means of communicating to others if one is attacked. Do you know of any research in this area

Answer
The fish will hide after the herons have visited.  I don't know anything specific about the grass snake (where do you live; I'm guessing not US since you used the metric system).  While some snakes do eat fish, a snake probably wouldn't make the fish cower because the snake could swim through the entire pond.  I don't know of research about fish communicating.  I think fish sometimes see a buddy being attacked or whatever and naturally know to go hide.  Good luck!

Robyn
fishpondinfo.com