Man alive, that all sounds complicated and expensive.. and L already wants to get rid of them to replace it with a hot tub, if I have to spend any more money on it she'll probably just let the cats eat the fish
We already have a big filter with a UV lamp in it, but that seems to do the square root of sod all.. we can try feeding them less - no idea how much of the pellet crap they're actually meant to eat, mind
just put in what they will eat straight away within 5 minutes
the UVc lamp (bulb) requires replacement once a year and cleaning on a regular basis to work
that's if it is big enough to cope with the size of pond
be warned, hot tubs require constant monitoring of ph, chlorine, alkalinity, TDS levels etc etc
what about astro turf ?
or a nice slabbed patio