I have a Brinsea Eco 20 and a Mini Advance (the mini is on its way to me, now). I only have one roo covering 30 hens, so my fertility is going to be pretty low....so my plan is to candle all 21 eggs in the Eco 20 on day 9, toss out those that are infertile, and replace them with more eggs....then repeat the process. The mini will be for those eggs that make it to lock-down/need to not rotate/need high-humidity. My problem is that I'm worried I'll forget which were put in on what dates, so I'd like to date the eggs on the day they were put into the incubator. What can I safely use to write on my eggs?