Purple up is a calcium and Iodine supplement. nothing more nothing less. if your not testing for both you shouldnt use it. Iodine is easy to overdose on. and too much calcium can throw your alkalinity out of whac.... its just as easy to maintian good water parameters without it.
a lot of the success storuies you hear are people go a few months and dont see growth and start using it then see growth. the thing is coraline often takes several months to hit a peak growth with or with out purple up. once people start to see growth they attribute it to the purple up, even though the coraline would have started growing anyway, there is no real proof that it does anything special other than get you to pay for water mixed with calcium and iodine... myself I dont like to pay for water. its cheaper in the long run to just buy calcium and iodone and the test kits for them, with out the water mixed in. even if you use Purple up YOU STILL NEED TO TEST for both calcium and iodine before using it. if one of the two isnt low DONT use it. you can kill or really mess up your entire tank.