The level of dissolved oxygen in water strongly affects the growth of organisms in aquaculture and profoundly affects economic in fishing industries. Continuing measure and monitor this level is very important. However, commercial sensors are expensive and not easy to apply to the aquaculture industries. Methods to measure the level of oxygen in the water have been developed long time ago using chemical, mechanical, and electrical principles. The optical principle has been recently utilized in this type of sensor in which Stern-Volmer law is used to estimate the quencher concentration. This work uses a very simple, low-cost, off-the-shelf MEMS visible spectrometer to measure the ratio of the intensity lights generated from the reaction when oxygen reacts to the fluorescent agents under excitation of the blue light. The sensor was built and tested against the commercial devices made by Extech (Model 407510). The heart of the sensor is the AS7262, a 6-Channel Visible Spectral ID Device made by AMS, and the fluorescent dye. Experimental results show a promising device which is very low cost, simple, low maintenance, and easy to use in the field to support aquaculture industries and environmental agencies.