Wrong bulb. Or rather, you need a UVB bulb in addition to this one. This is a basking bulb, which emits heat. The description also says it emits UVA rays, but there is no mention of UVB rays.
Regular UVB lights are flourescent and don't emit heat, but do emit those UVB rays. (In the future, you can just use a regular lightbulb for heat in addition to the UVB light).
If you need a UVB light for a ceramic dome, get a Reptisun compact flourescent UVB light (they come in 5% and 10% UVB strengths, since your turtles haven't had a UVB light, I'd get the latter. You can get them fairly cheaply (compared to buying in a store) at
www.bigalsonline.com (type the name of the bulb in the search function to see and read about it). The bulb should be positioned 6-8 inches from the basking turtles, next to the heat lamp (the Zoo-Med one in this case).
When the weather is nice, it would also be good to take the turtles out for some natural, direct sunlight (glass, etc. blocks UVB rays). If you put them in a container, make sure it's not glass and that they have a shady spot to go to if they want to. You could take a newspaper/magazine outside and supervise them for the time they're outside.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-