My galvanized well tank shows 70 psi on the gauge, but water pressure is low. How can I fix?

If your pressure gauge reads 70 lbs you should have 70 lbs of pressure on the line.
Open a faucet and watch the pressure gauge. Pressure probably drops dramatically and pump comes on immediately. When you close the faucet the pressure will probably go back to 70 lbs and the pump will immediately shut off.

If these things are true you have a "water logged" tank. There is no air in the tank to compress. It has all been absorbed by the water. You cannot compress water.

The volume and pressure you are receiving at the faucet is what the pump is producing while the faucet is open.

Turn off pump. Open the drain line at the tank. If you do not have a drain line you will have to open the line from the well. Open a faucet to let air in and let the tank drain. When tank has completely drained, close drain line and faucet. Turn on pump.

This is a procedure that must be done periodically with a pressure tank.

I would also check the piping, sometimes they get clogged with rust and buildup. You will have pressure but little flow.