Not just "once". Many conspiracy theorists still believe that it is some form of government plot or another. The theories range from it being a toxic waste disposal scheme that is poisoning people (and as far as I know, there is some basis for the claim that fluoridation came about as a way to cheaply get rid of a relatively toxic byproduct; that doesn't validate any other part of it though), to fluoride being used as a mind control chemical (usually pointing to drugs like Prozac that include fluoride as "evidence").
Of course, none of these "theories" manage to address the fact that there is fairly strong evidence that water fluoridation does in fact reduce tooth decay.
Of course, none of these "theories" manage to address the fact that there is fairly strong evidence that water fluoridation does in fact reduce tooth decay.