I personaly and my plumber agee's I would have used a bladder or diaphram tank instead of just a storage tank. It's common to see a pump come mounted to a storage tank which is fine but every time the taps opened it turns the pump on and will wear out the pump and contacts far sooner.
Your tank is suppost to have some air at the top of it to act as a cushion to stop water hammer but to often I've seen them become water logged.
He's some info on how they work.
If it was a bladder or diphram pump there would have been a gauge on the tank and an air valve to add air. It's set to within 2 to 5 PSI of what the pump pressure is. I would not add air to your tank. You can however drain the tank and refill it which shoud give you the air space back.