Yeah, I do that too. It's a little more time consuming, chasing those teeny babies around and dragging them to the Sale Tank tab, but it's worth it in the long run. Only when they are getting close to maturity, do I even have to worry about them while in the Sale tank, because they don't sell until they reach maturity.

I try to buy the extra tank as soon as possible, so that I can keep one tank for breeders, one tank for keepers, and the Sale tank for babies and sell-offs.