isolators, batteries, power cells

i have a 2nd battery in the back of my truck. wwit size isolator do i need. and how do i hook it up. i have the batterys run parrelel and im thinking of adding a 3rd battery i jus bought somemore speakers and my headlights started to dim. i have a 10 farad capacitor but my voltage never drops below 12volts

im looking into buying a power cell in replacement of my 2nd battery. i was looking at the kinetic brands. im not sure what size i would need i have about 10,000 watts of subs in my truck. how would i hook it up. my batteries are run parrlel now

"i have about 10,000 watts of subs in my truck" what do you mean by this? Amp power or some numbers printed on the speakers somewhere? If its amp power, what brand of amps are you running that's 10K? If you aren't running multiple alternators already I HIGHLY, doubt you are running that much power. If the 10K are speaker numbers that number is meaningless at this point. No matter how many batteries, if you can't charge them your wasting your time. Before you can get a honest answer we need know exactly what your needs are, not fictional numbers on either the amps or the speakers. We need to know year, model, gear etc...

ive got a 2000 chevy suburban with 2 15" l7s n 2 12" fusions plus 8 6.5 " fusion speakers all totaling just under 10,000watss total, all amps match rms of each speaker but i have it turned way down cuz its so lovein loud. everything is run with 0/1 guage wire.