I know the problem. living in a pretty rural area we have big problems with the Heron's depopulating the local ponds. I built my pond around 15 years ago, it's oval in shape, 10 feet long and 6 feet deep at it's center with a natural stone wall around it about a foot high. It has around 40 fish in there comprised mainly of Ghost Koi and a few goldfish the last neighbors sonated before they moved away plus some fish my brother caught when he was younger. the Heron tried for month's to get at the fish but the high stone wall prevents it from getting at them. I'm lucky in that respect, the design prevents the Heron from getting anything except bored! BUT a lot of theneighbors have ponds and aren't so lucky. one guy goes out at the end of each month and spends a fortune on fish, simply to replace the ones the heron has eaten over previous month.
Whilst you have a virtually empty pond it may be time to look at preventing the heron from getting the rest of them. I've built a few ponds in the intervening years and can offer advice if you like- i did'nt know you had fish or how much experience you have with them
