A lot of times low pressure in a household water system can be from a small piece that is put into the showerhead/faucet to restrict water flow; this is usually found in newer homes or bath remodels.
Well to adjust the water system, you need to install a new tank, one that actually has higher pressure. You can get the parts and replace them as required for you.
Call the water company and see if they can send someone out to come and take a look at it. There are lots of things that could cause this to happen to your water system. The professionals have the correct equipment to check it out with so you are better off using them.
