Calculating Current Capacity on Fuel Tanks

Hi Everybody - First time poster!
I created an excel spreadsheet to track transactions that either add or remove fuel to/from a tank or trailer.
I want to show the current amount that is in the tanks on the first sheet. The transactions will be listed on the second sheet.
I'm not sure how to calculate the formula to show the current level in fuel tanks based on transactions since I'm transferring from one to the other and in some cases just adding or removing amounts. I've attached my basic spreadsheet.
Any help you can provide is appreciated.