i dont candle mine so i wouldnt know, and on the 18th day the humidity needs to be turned up, if all goes well then they shoul hatch on the 21st day (preuming they are chicken eggs) and what temp and humid have you got them on??? (:
if the incubator is manual and not automatic then usualy afew days before the hatch date you stop turning them, and if it is automatic with a rocker/cradel then it is to be switched of ytree days before, hop this helps
here it is, it hold 25 chicken eggs and is £150 ish, very good
http://www.ebay.co.uk/itm/Incubator-RCOM-Suro-24-Egg-Incubator-Hatching-Egg-NEW-/310243128228?pt=UK_Pet_Supplies_Poultry&hash=item483bee73a4