I work with animals and what we do when we come across tanks (donations) is scrub them down with dawn soap, maybe some bleach in a bucket, and scrub pad too, (this process may take a few hours, to even a few days) Then Drag it outside and Hose it down for 10 to 15 minuets, bleach the entire thing for 10 minuets, scrub any stains that have stuck again, Hose it down again for 10 to 15 minuets, Drain it, and Use 1 part vinegar to 3 parts water, (I make it strong) drench the thing for 10 minuets, inside and out. (use a mask, this Can make you dizzy) Rince it for 10 minuets, THEN use a reptile safe cleaner for the last step. (usually I let the tank dry first, then use the reptile cleaner) but ONLY if you are SURE you got all the soap out of it.. if you arent, then Start hoseing it down again, run your fingers over any area you think you might have missed, if it comes up with little white spots, or suds.. then you havent cleaned it good enough, and you may have to take a scrub brush to all the little corners between spraying it down with the hose. you can use windex, as long as you rinse it clean after, Id use reptisafe after everything is done though.
dont take short cuts with dirty tanks. this may take upwords 3 to 5 hours.