💡Lesson Learned: Waterproofing temperature sensors yourself as shown below is not worth the effort, as you'll find stainless steel, very well waterproofed sensors on eBay searching for "DS18B20 stainless" at a reasonable price. I switched to these after one or two of my sensors failed.
Below is one of many DS18B20 temperature sensors used with my controller to read the temperature in the aquarium. Since these sensors are relatively cheap I used a lot of them to record water and air temperature, waterproofing the sensors myself. Using as many sensors as I used is overkill as one accurate water temperature reading is probably sufficient for most aquarium needs, but I thought it would be useful to display more readings on the LCDs, for example, to know if the hood above the aquarium was getting too hot from the lights.
Below is a short video showing a test of a temperature sensor and readout on the LCD.
For the final heat shrink tubing on the sensor, I used aquarium safe super glue as a waterproofer. As noted above, it's not worth doing this yourself as you can buy very well sealed sensors cheaply these days. Also, I would caveat that I've replaced at least one of these sensors that did not withstand being submerged, so this may not be the greatest waterproofing technique!
Sensor wires were then bundled together on d-sub connector to later be plugged into the controller box.
I also labeled the sensors so there was no confusion later when I deployed them in the aquarium.
