I just switched to a 10 gallon for my betta a couple weeks ago, but I still have his old 1.5 gallon that has been just sitting there for 2-3 weeks with water and gravel in it, but no tank upkeep, filter, heater, live plants, etc. I emptied it out and besides one little rust-colored spot right where the water line was and some calcium deposits from the hard water, it looks completely clean. Is there any kind of disinfecting I should do before I use it as a holding tank for my cories while I clean the big one? I would leave it empty most of the time, then fill it 4-5 inches full with the tank water from my old tank.
Oh, and is there any reason not to do 50% water changes with the cories? I know the Betta doesn't mind, but would the cories be able to take such a big change?