If you have a 10gal, yeah, use one of those. What's your room temp? If the water in the 29 is consistently staying around 74-78 F, then take the heater from there. The danios and cories can tolerate lower temps like that easy.
EDIT: Can't treat the whole tank with the cories. They can be salt-sensitive. Also can't treat the whole tank if you have live plants. Live plants melt in saltwater.
Watch them for a day or so. If no one is darting or rubbing and the white spots disappear, you'll be fine and you won't have to worry about treating.