I used a cut down styrofoam egg carton for my first hatch. It really helped keeping the hatched chicks from kicking around the other eggs that had not hatched yet. At day 18 you should be able to candle and really tell the clears even on the darker eggs. I would try to get as many of the non starters out at lockdown as possible. If you are just not sure, then leave them in but take out any that smell or are seeping.