What is the character of the feature. Is it a pond or is it a drain or what. Not sure how it could be kept stocked with chemical if it is a drain. If it is a pond, how about turning a lemon into lemonade with algae-eating fish.
Lanthanum is reportedly a low toxicity chemical, but if it got into a cow pasture and was assimilated into vegetation the cows eat, there could be issues for whoever consumes the milk and/or beef. A bitterant could be added to the water to discourage dog drinking.
It is a finished water feature sort of a fountain. Concrete, stone lined, filtered.