I haven't used a UV Sterilizer on a fish tank....Growing up my family raised koi in the backyard. We had several ponds (my parents paid their young son) to dig for them during the summers. Note: $5/hr is good money for a kid if they are only investing in candy-corn and gummy worms.
Initially we had a big problem that we couldn't see the fish in the water because the green algae was too thick. Our filtration systems didn't seem to catch the free-floating algae. We installed UV sterilizers on the outtakes of all of our ponds, and the clarity of the water cleared significantly within a few days (the algae on the surfaces remained). Unfortunately the clear water invited predators to come visit our ponds and even with netting on the surface the great blue herons were relentless at trying to eat our smaller fish, we eventually deactivated the sterilizers on all the ponds except for the display near our house.
My parents stopped breeding koi due to an extreme winter in which we lost several of our older/larger specimens--the larger the fish became the deeper the water they required to make it through the winter, and we found out the hard way that we did not have adequate depth for 3 foot long koi...
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