I have a drilled well with an expansion tank. A bath RC shower head is under 3gpm wide open. Sounds like the existing air pre-charge and cut in/ out was not adjusted correctly in your system. The drawdown volume in a bladder tank is only a small fraction of the tank gallonage - most of the volume is air.This is more to vent than anything. Today I was taken advantage of big time. Long story short, I moved into my current house 3 weeks ago. Have had an issue where I lose water pressure from my well. Turns out my well only does 3GPM. Installer recommended a second pressure tank to supplement so I have more buffer. The installer was recommended by a family friend and he knows people I know. He installed a pressure tank and pressure switch for $2100. Over $400 of that was labor, so I guess he charges over $200/hr since he was here two hours. Attached is the invoice for your amusement.
Is this a 2 story home and was this issue on the second story? Do you water your lawn? What water pressures were you seeing?
Adding a second tank reads like a really bad solution, as if you are using near the 3gpm supply rate, pump will just cut in at say 40psi and never make up after the cut in as draw and fill are at the same rate. A filled expansion tank should handle a toilet flush. or about 10 mins of doing dishes in a sink without a pump cut in.
I would like to hear more about tests the plumber performed before he made recommendations.
I "allowd myself" to be "shafted" by a well company a few years ago too when my pump died. Long story, but you become really desperate when you have no running water for a day or so and then the crew arrives and says they can start the job in and hour, you feel "saved" like a damsel in distress and just open that cheque book lol.