Thanks Guppy,
I suspect the garden police won't allow anything like that. The steel grille is a temporary measure while I see whether I can put a framework in for a sub-surface plastic grid. But either way I'd like to be certain what's killing the fish first.

I live in S Oxford and have a wildlife/Koi pond, kidney shaped- 7m x 2m (max) x 1.5m deep. About three years ago it was stocked with small koi and three green carp. The latter are now 30 cm long and live at the bottom, the koi have disappeared probablt due to a heron that is a regular visitor despit...

Thanks pacman75,
Yes I guess that's an option, and might be more effective in the long run.

Hi, I’ve been filtering my 4000l garden pond with a Hozelock Bioforce Revolution 6000 filter and Aquaforce 4000 pump for last 4 years. Last week system developed an electrical fault continually tripping RCD. Seems to be the UV wiring in the filter (which only has a 2 year warranty) and there is no p...