Seafoam, although many say to use it in the gas tank, I have never noticed any difference. When I have noticed a difference, I put some seafoam into the oil, right through the oil fill(changed the oil about 200 miles after I did that) and I also put some seafoam into a vacuum hose coming off the crank while the car was running(i.e. seafoam was in a cup/container, took vacuum hose off and while the car was running stuck it into the container with seafoam so it sucked it all into the crank(case)) some people say to let it suck in seafoam until it stalls out the car, I would just let the engine suck it up and then turn the car off and let it soak.
I do not recommend leaving seafoam in your engine for very long. AFAIK there are no actual studies (real ones) done on the effects of seafoam on the internals(seals, gaskets, things that can deteriorate, etc). I am not going to put this into my brand new car because of this. If you want to be a trailblazer go ahead.
Also, I did the above on a now 15 year old car and it worked great as far as the car was idling bad before and there was definite buildup of carbon causing issues.
I have no evidence to back any of this up except google the procedures I listed above and it is pretty common. Once again, in the gas tank I dont see how it would clean anything except your gas tank and fuel lines, might even step up the octane lol.....