I have an idea for a temperature sensor well. The storage tank is 1200 Gallons, horizontally. 19.5 ft long by say 40 something inches. Use a long temperature sensor well on the vertical something that almost reaches the bottom. Place the sensors in the well at the heights you want, then fill the well with transformer oil to help conduct heat. The same stuff used on pole transformers. It's nonconducting electrically so the sensors should be fine. You only need one well and can place the sensors at any height. I was thinking something 3/8" in diameter for the well. The top would have a compression fitting to keep water out. Thoughts?