What are you trying to clean from the inside?

Why is he trying to clean from the inside? Over time dust, contaminates in the gas, Grass, ect.. get into the tank. Gas jells this stuff, it sticks to the bottom, sides, and around the any exits. Then it harden or just floats around the bottom that don't get stuck.
I find or IMO the every 3 to 5 years depending on the conditions the gas tank gets pretty cruddy. When I do Fuel system clean up I start at the Tank and work my way to the carb. I know some people just change the inline filter thinking that's all that needed. IMO it's not.

Cleaned many plastic gas tanks with carb. this includes weedeaters, chainsaws, lawnmowers, Lawn tractors. My my 23 year old craftsman LT4000 plastic gas tank has been cleaned at least 10 times. I usually spray the carb. cleaner in, use bottle brush for the corners and dirty spots on the bottom. Then I blast it out with my pressure washer. Rinse, flush with gas, and let dry. This I inspect, blow out all the vent holes (including the cap, Course I cleaned it also). IMO you can use several chemicals including gas as long as you use the bottle brush, rinse and alow to dry out. I just found that carb. cleaner is more active initial before it evaporates. If carb. (in spray can hurts plastic then all the Zamma carb. would be melted POS cause I spray the shyt out of them to get them clean when they have been setting for year or more.

somewhere in the process, try hot water and a strong detergent like Simple Green . So Far I havent felt the need to mess with any plastic tanks

I haven't ever soaked a tank w/ decarbonizer, but if I did- time would be minimal. 15 min may be too long, I dunno. The spray carb cleaner (Xylene) is helpful.
Andyma brings up a great point. People are afraid to use water in a fuel tank for some reason. In fact, once you supply cans are empty, open them up and let them dry. Then give them a wash and rinse and once again let them dry. You simply get krap from the pump that is going to go somewhere. It is also a good time to clean the fuel cap, area around the tank opening plus tank on the equipment if you are up for it.
Goes hand in hand w/ what rc is saying. Your freshly cleaned carb wont stay happy if you feed it dirty fuel. Looks like Ross saw that coming!

Ross: My 2 cents any Concentrated Fuel Cleaner should suffice . For really stubborn cases a bottle brush as advised will help with any build up within the corners or edges. Flush with fresh fuel and dispose of in a safe manner. Using a Green Formula and air drying sounds like a valid scenerio for milder applications or in conjuction with carb cleaner as a final rinse. Even spray washer usage would work if your so inclined ensure to use safety goggles in this form of cleaning . Have cleaned many motorcycle , snowmobile and snowblower and lawnmower (plastic) tank via Fuel Cleaner addition no harm observed.

Sometimes I walk a fine line with some customers about why their carb bowl and jet are blocked with dirt, especially a few days after I cleaned the entire system. They want me to re-clean because they don't realize their gas can is dirty. I do it once and give them the warning.

Isn't it the truth lol :(

A neighbor had problems with dirt and water in his Craftsman tractor, so i showed him what and how to fix it. He used a tin gas can, galvanized, and would let it set out in the hot sun when not mowing. This caused condensation to form, which we all know as "water"!
Then, after he died, his son would stop in and use the tractor. He decided it wasn't cutting level, so he adjusted the wrong things, which only made it worse. He told his mother she needed a new machine. (Ohh, yeah--his father died, but came back to mis-adjust the settings on the mower deck!) So, after the grass grew back in, i set the adjustments back to the correct settings, and she's still using it.