superfluoridous

When you add an insignificant amount of something to water, thus failing to have the desirable effect. Or when the effectivenes of said additive has peaked for some reason and using more is redundant.

Derived from the words 'superfluous' and 'fluoride'.

"My gran always uses a superfluoridous amount of cordial ; it's pretty much just coloured water."

"The fish tank is already algae proof, so any more steriliser would be superfluoridous."