I use IR gun thermometer very accurate, the thing is when taking water temp the beam goes through the water and gives you the bottom of the res temp , So if you have a cold floor ,your temp wont be accurate, you will get a accurate water temp reading if you shoot the airstone ,or keep somthing solid in the bottom of the res and you will get a accurate reading , what nice about it you can shoot anything you want like your medium ,like hydroton,RR and you can see the differentals from your water temp ,also you can shoot evey inch of your grow room and see the temp diffs ,you would be amazed at the temp difs from the bottom of your plant to the top, here is an example My 400w MH bulb temp is 286 def f when in full operation. for me great tool , shoot my soil temps too.