I tied my drop checker, with a short length of string to a stone, so it could be moved around the middle of the tank, rather then be just stuck to the glass. Using my 1500l/hr filter + internal 600l/hr filter in a 180l tank I found spots where the drop checker was still blue(ish) when lights came on, compared to a nice green/yellow in other parts of the tank.
A 3200l/hr power head fixed that issue.