I put a UV on my pond this year and I was happy in a little over 24
hours. I have a 30 watt uv on 1350 gallons. My pond is full sun--all
day. I get some floating algae on hot days, but it goes away at night
or I skim it off. I notice some variation in the water during the day
as the algae tries to grow but the UV catches back up at dusk. I
think my $230 investment was well worth it.