The bladder seems to be leaking. Water pressure in the house was getting weaker, and the tank was switching on too frequently, so I switched off the power and drained the tank. After powering back up it seemed to fill about 1/2 full and pressure in the house was restored to about normal...for about a month. I believe I just need to put in a new pressure tank but HOW?

The house and tank are 8 years old, and here are the specs of the current one:

Do it Best
Well System Tank (about 36" tall, 15" wide)
Factory Precharged Pressure 28psig
ID #16231429
Max Working Pressure 100psig

What kind should I get and what directions do I follow to replace it?