I respectfully disagree with the temp recommendations above.
Diamonds love heat that mimics the heat of the sun. In indoor captivity this should be a light emitting globe, positioned above a perch or shelf. The heat needs to come from above, not below (i.e. no heat mats etc).
Instead of having a hot and cold area in the tank, the heat under the globe should hit around 30 degrees celsius but only for 3 to 4 hours a day, preferably in the morning. After that, all heat should be turned off for the rest of the day and night (unless you live in a really cold climate in the mountains or something).
Having an area in the tank that is 30 degrees all day will likely be causing the problem you are seeing in your diamond at the moment.
Just my 2 cents.