Here is a YouTube video that does an excellent job of explaining the process:
My tanks are 16 feet to the pipe opening to the bottom of the tanks, so the core sampler in the video does not work well. I am making an improved version that uses a one-way value on the end of 10 feet of clear PVC pipe (the actual liquid in the tanks are 9 feet deep.) I will have a 7' screw on extension of just regular PVC pipe.
Here is the valve I am using. I made another core sampler using this model valve and it worked very well.
I bought the clear, schedule 40, PVC locally for about $55 and the fittings and regular PVC at Home Depot. The whole thing costs about $80 and, most importantly, I get to screw around in my workshop a little -- and that for a core sampler that will work on my huge tanks.
Compare that to the cost of pumping too little or too often.