I take care of a couple small fish ponds and that's the way I've done it without any losses. When possible I like to introduce fish (especially big ones) to a new system 1 or 2 at a time until I know the system can handle a little more load. It makes sense what your vendor is saying because size of fish plays a big factor. So when do the fish arrive?I eventually got tired of the extra step and just started adding Prime directly to the tank before refilling. It's always worked fine.