Yes. The water coming through the python is untreated, so you need to dose Prime (or whatever you use) directly into the tank.
We do have to be careful not to suck up fish or other critters when gravel vaccing or draining water. If that does happen, though, there's a quick shutoff valve right at the end where you're using it. Turn that off and watch the snail or small fish float back down, safely. You have to get used to using these.. and never leave it going without paying attention.
If you have a sand substrate, it's also a good idea to place a basin in the sink so that won't go down your drain if you accidentally suck up some sand.