How can I keep my 6 gallon tank from collapsing in between fill ups??? I do vent the tank from time to time, should I not do that. My tank was almost empty last weekend and I thought filling it up would round the tank out but I was only able to get 3-3.5 gallons in it. I usually let my tank run down before filling up so I can have fresh gas/oil mix. Any info is greatly appreciated.