If I were you, I'd continue with spot treating with Excel. The amount you're dosing should allow you to treat the entire tank. Maybe up your water change frequency a bit during the treatment phase.
I'd also try using peroxide to treat. That seems to always do the trick for me when Excel isn't cutting it. Your fish are tough so they can handle it if you use small amounts.
After treatment is over, I'd ditch the Excel entirely and just use root tabs and the occasional dose of ferts as needed. The plants you've got will pull most of everything they need from their roots.
About Otos = were you feeding them? Often, there's not enough for them to eat in a tank and they'll require blanched vegetables or something equally suitable for their diet.