On thursday last week I noticed that my normally friendly fish had not been to the surface for a while. I didn't think anything of it untill the weekend when they still hadn't shown. They had not eaten any food so I was geting worried, I was thinkiing that someone had nicked them or the local heron had had them. Usually they would at least dart from one peice of cover to the next, but nothing. I went out on the saturday night and shone my torch into the water and there they all were right at the bottom of the deepest part of the pond milling about.
There has been a lot of rainfall recently and the pond overflows when it is full, could it be due to a change of water quality?
There are a lot of tadpoles in the pond could they be gorged on these?
They have been spawning during the last month could they be exhausted?
Any ideas as to why they still haven't come to the top for food yet?
Yes, you have partially answered your own question.
The first thing that came to mind is that they were badly spooked recently. You did mention a heron? A couple of near misses can send them flying to all corners.
Water quality is the second on my list. A sudden downpour can play havoc with WQ. Especially if the rain is acidic and you have a sudden drop in pH. You need to do some waterQ tests (ammonia, pH and nitrIte for starters, nitrAte and KH/GH would be nice too). You need to establish your normal baseline and then you will know if you have a sudden change.
Try giving them their favourite food. Rub some garlic on your finger and on their food and see if they come out. See if you can set up a jet stream with your pump. Sometimes they like to play in the currents.
I Can't say that I've noticed any scum. I've heard that herons like to stand level with the water and fish down. My pond is semi raised the lip is about 2' higher than the surrounding area with an area in the middle that goes down to about 4'. I have never seen herons in my garden although they do fly over on a regular basis. Another thought is that next door are having an extension built and the builders have been using a Kango perhaps the vibration from that has upset them.
Have you tried putting up heron fencing? We have put it round our pond just to be safe. It is the plastic cord, 3 tiers of it. Quite descrete but enough to stop a heron finding anywhere to land. Not sure if you can see it in the photos of our pond? Our local herons certainly haven't succeeded in taking any of our fish.
My pond is semiraised, knee high, and believe me heron will fish from outside the pond, inside the pond or on the edge. Once they feel they know the water and have checked it out they will also land in the water. To get into my fish pond they have to, there's a electric fence around the pond so thay cant walk in. They hop over the fence. The fence has been off recently because I need to remount the posts, so they dont get a shock if they touch it. When the fence was ON it did seem to stop them. My pond has shelves at 1ft and 2ft and a deep of 4ft herons will stand in the 2ft section but I am not sure if that is on the liner or on the plants and or 'pots'. The fish are in the 4ft at the moment though how many there are I dont know.