Shell rot is a problem that faces many diamondback terrapins. Shell rot are soft spots that develop under scutes and if left untreated, it will spread all over the plastron and even their carapace. It normally starts on the plastron. It is very common for wild caught diamondback terrapins to develop this problem. Dirty water, basking problems or a lack of salt in the water are all factors that can cause shell rot. Also if loose scutes do not come off they will trap water and bacteria between the scute and their plastron will also lead to shell rot.
The first part of treatment is removing any loose scutes and the soft spots where the shell rot developed. It is very important that all of the shell rot be removed. A toothpick can be used to dig out any hard to reach rotten shell. This is an unpleasant experience but completely necessary for recovery. Digging out the shell rot might cause some very minimal bleeding. Once the plastron has been cleaned of the shell rot cover the plastron with Betadine and dry dock the terrapin in a warm area for at least 48 hours. The Betadine helps prevent infection.
I have tried many types of treatments for shell rot and none have worked as good or as fast as simply increasing the saline level to 1.017-1.019 on a specific gravity meter. At this level the shell rot should stop spreading within a week or two and should start healing fairly quickly. It will take about 3 cups of rock salt per 10 gallons to reach around 1.017. The only draw back is they will not be able to drink the water and will need access to fresh water once a week. After three months if the shell rot is completely gone the saline level can be lowered to between 1.012 and 1.014. I would suggest keeping the water brackish for a couple more months.
As a preventive measure I would suggest keeping any new wild catch arrivals at 1.018 for the first three month and then I lower it to 1.014.
Common example of shell rot on a plastron.