Re: I got BAD fuel in my below deck tank.
first we need to hear back from OP about why the gas is "bad."
If it is "bad" such as phase seperation or kerosene smell, adding sea foam, stabil, or fresh gas won't fix it.
if there is water in the tank, additives won't take out the water. . For one thing, the water is at the bottom, first thing the tube picks up, so no matter if there's 5 or 50 gallons on top of an inch of water, there is still an inch of water. And salt water is worse. And I wouldn't rely on a water seperator to handle it, either, if I knew it was a lot; they are designed to keep out some water but not to clean bad gas.
Adding fresh gas to bad gas won't make bad gas fresh. It will, however, dilute the "badness" to a point where your motor might not notice. So if I had really bad gas (my wife says i do) then I would remove it. if it was marginally bad, or just old, no kerosene smell, I might dilute it.
one year old gas in Texas can be bad due to heat, humidity and temperature fluctuations, but not necessarily.