I believe you should carry out MicroTiter Plate (MTP) assay by adding 200 µL of a 0.1% solution of crystal violet to the microtiter plate where there's preformed Biofilm on ELISA plate. Afterwards, to quantify the biofilms, 200 µL of 30% v/v acetic acid solution should added to each well to completely solubilize the crystal violet. The resultant solution will then be transferred to a new microtiter dish and absorbance reading was taken in a micro-plate reader at 595 nm.