Just installed a Stenner pump with a gray 15 gal tank. Works great so far, but does anyone have a good method for measuring how much liquid is in the tank? I have dumped about 9 gals of bleach in so far, but would like to know how much capacity is remaining. I tried shining a bright flashlight into the fill opening but still cannot see how much is in the tank using the graduations on the tank. I suppose some kind of dipstick would be in order. Has anyone made one that is easy to use? Thanks.