Methylene blue used as funguscide in cichlid reproduction - concentration measurement?
The concentration of an aqueous solution of methylene blue can be determined using a UV-VIS spectrometer. 

Measure the absorption at $\lambda = 664\, \mathrm{nm}$, use the molar extiction coefficient of $95000\, \mathrm{dm^{3}\cdot mol^{-1}\cdot cm^{-1}}$ 
(Reference) and calculate the concentration using the Lambert-Beer equation.