thanks for advice neilsignum.
took most of it onboard.
its a 25w uv am using. the filter is 5 stage kockney koi mega xl.
pump is PISCES INTREPID 12000
Flowrate: 12000 lph (2600 gph)
Flowrate at 1m lift:10000 lph (2200 gph)
Max lift: 5m
the pond was strengthened with rebar from top to bottom, and 1ft apart, it was then sheeted with ply to spread the weight of water.
also fitted retro bottom drain so it should keep the water pretty clean. New waterfall is 900mm long stainless thing so will help with aerate the water plus putting air pump Max pond volume: 4500 litres (1000 gallons)
Max depth: 1.4m (4ft 6in).