I only candle the light color eggs, like while, cream, light green/blue. I used to candle the brow, dark green, but saw nothing. Maybe I need a brighter bulb.
For the light color, I do it on the 7th day.
I got a great little LED flashlight at TSC, its only about 3" long its super bright and works great to candle the eggs. I have not tried it on the Marans yet, but works great on my blue eggs. If I do not need room in the bator, I don't candle till lockdown, unless the bator stinks! If that happens, I will very carefully candle the oldest first (I've been doing staggerd hatches) and keep going till I find the bad egg. You will know by smell if you have a bad one. Good luck, and have fun with your babies. Nothing better than a bunch of brand new fuzzy butts.
Im pretty good at candling now and i used this http://shilala.homestead.com/candling.html as a reference when i candled them the first time its pictures of bad eggs and good eggs
So it has helped me alot with candling and you can pretty much candle eggs day 4,7,14,18 but i candle them only once and thats day 18 when you stop turning them cause then the egg is fully formed and for my candling i took a tin can and punched a hole the size of a quarter through the top and you set the round part of the egg on it and take a flashlight and shine under it you should see if theres a chick in it and if you open the lid and you get a faint smell then you have bad eggs,I know what mine smell like my first time around it was pass day 21 and i kept smelling something awful and my mom was saying its in my head so i couldnt take it anymore and i buried the eggs and cleaned the incubator i came back in and the smell was gone