Not only it will not work in water, but even in soil its indications are dubious. Results of measurement in this type of device depend on the soil conductance so they change with the soil humidity. Not recommended. As fresh_newby stated - go for a real one with glass electrode if you want to be able to measure pH with reasonable accuracy. And even then to be sure you know what is going on in your tank you need to calibrate the electrode before the measurements. pH measurement is not a rocket science, but you have to use good equipment and follow procedures, otherwise it is better to use pH strips - they are not as accurate (mainly because they are often difficult to read) but they will never give large errors without warning.