What is the best way to clean a 300 gallon Water tank that was storing Canola Oil?
Hot water (how hot would depend on the type of plastic - at least warm, to start with) and baking soda (bicarbonate of soda.) Let stand (or agitate, if you want) for 24-48 hours, repeat as needed. 

If you can source food-grade lye and are aware of and able to take appropriate precautions for handling it, it would work a bit faster, but in both cases you are using an alkali (mild or strong) and that will saponify (turn to soap) the residual oil. Baking soda is cheap, safe to handle, and easy to get as a food-grade item.