Low water level in house water tank

I have a problem with my water system. I have a submersible pump and when it kicks on it runs up to pressure as per the gauge and shuts off, but it hardly fills the tank. It thinks the tank is full. I can hear gurgling of the water coming into the tank that I never did before. I have a tank with a bladder with 35 lb air pressure. There is hardly any water in the tank, it is so light you can tip it over with your finger, so any time you turn on a faucet the pump comes on. I also have a diatimatious earth filter system connected to the tank and I keep getting air in it, and it blows away the bedding so I have to keep replacing it. I think the trouble might be the tank but I need advise on how to prove it and what to do afterwards to clear it.