I am doing away with my 20 gallon tank for a Rio 180 which I am going to cycle for about a month with no fish. If I mix in some sand with my nutrient substrate of choice (if I ever I choose one) from my well established 20 gallon tank will this help the cycling process even though it will be under 3 cm of 1-3mm size quartz gravel?

If somebody could also explain how it does help that would be great.

Cheers
Jon

Check out my Guide. There is a section on cycling a new tank.

If you add old substrate to the new tank but don't give the bacteria something to live on they will die. So you would need to have a source of ammonia in the tank.