painted turtle-we think

Question
so my grandson age 10, founf his turtle laid an egg in the water aquarium-- the turtles 90 gl home.  the question is what to do with it?   the size is approx 9" long.... they have 4 bigger turtles all swimming in the water, or up on rocks and the one end does have a heated lamp.  of course he called grandma and grandpa.  any suggestions.   He told me "the father turtle is watching it closely.  Help

Answer
The size of what is "9" long"?
For the sake of these many turtles, I hope this is a very very wide 90 gallon tank. 4 turtles is a bit much for an indoor tank unless it's well over 100 gallons. An outdoor habitat is recommended. A heat lamp is also not sufficient. These turtles need UVB lighting as well or you are asking for metabolic bone disorder. They also need dry basking area. There is so much more to know about turtles than throwing them in a tank, and for that reason I never recommend turtles or other reptiles to children. Actually, I find that most adults don't care for them properly.

The "father turtle" ( likely mother turtle or other female ) may be watching it to eat. If only one was laid, and the mother did not attempt to bury it ( if she had a choice of dry habitat ), then it is probably infertile. In the wild infertile eggs are usually eaten to protect the nest from predators who may smell the egg as it rots, and give away the location of the viable offspring and put them in danger.

The egg should be marked on the very center, top side, with a small dot. The egg must remain oriented exactly the way it was laid at all times. If it has been turned it will kill the embryo. You can carefully hold the egg up to a strong flashlight and "candle" it. If it is fertile then there will be a network of blood vessels and a dark mass visible within the egg. If it is fertile, the egg should be placed in a shallow styrofoam or plastic cup filled with damp ( not soaking ) vermiculite, by making a slight depression in the vermiculite to sit the egg. This container should then be placed in an incubator that you have purchased or made from plans, or placed in a styrofoam cooler in an un-airconditioned room that remains in the optimal temperature range. There are plans on the internet if you websearch. The egg should be monitored for temperature and humidity ( about 82-84*F and about 75% humidity ). Too hot will kill the embryo, too much humidity will cause mold that may kill the embryo.

There may be more eggs in the following days, whether fertile or infertile. Eggs that are not laid at once, or within the same day or so are probably infertile, but living arrangements, lack of suitable nesting area, lack of UV lighting, and other factors may make a hardship situation for a mother turtle to lay them properly.

Alternately, you may find an experienced turtle keeper in your area who can take the egg(s) if they are fertile, and that is really the best course in my opinion. Hatching, and caring for hatchling turtles is not something that I recommend for children to try, at the expense of the turtle.    

http://www.anapsid.org/mainchelonians.html
http://www.allturtles.com/setup/indoorWT.php
http://www.chelonia.org/Articles/chrysemyscare.htm