A lot of fuel tank cleaning depends on what the actual problem is.
If it is dried, crusted and rotten fuel, then something of a solvent is the best thing. Carburetor cleaner (the real caustic stuff in a 5 gallon bucket) works for this to dissolve and remove the old fuel. Follow this with a good detergent and you should be fine.
If it is rust, then the best thing is a weak solution of phosphoric acid and hot water to clean the inside of the tank converting the rust to a black surface ready to be sealed.
There is also a process using washing soda and a coathanger or bolt, along with a battery charger to remove the contaminats via electroalysis. The coathanger, or bolt in some cases acts as an anode and the tank becomes the cathode. It works, but take a bit of vigilance to keep changing out the coathanger/bolt as it gets contaminated with the material removed from the interior of the tank.
No matter what, muriatic acid isn't the best choice. It will work, but almost the instant you drain the tank, it will begin to rust that quickly. It is very caustic, and will find any weak spots in the tank. Neutralizing it is also a problem as well.