To use brackish or fresh water is an subject of great debate among the diamondback terrapin community. I will try to present the information I have gathered and my personal experiances. As I mentioned in the shell rot section, diamondback terrapins are very susceptible to shell rot and skin issues. If they are housed in fresh water their chances of developing shell rot and skin problems are greater than if they are housed in brackish water (1.012 to 1.018). I have broken down several different groups of diamondback terrapins and how living in fresh water can affect them.
Captive born hatchling diamondback terrapins can generally be raised in fresh water without any problems. It is always important to keep a close watch for shell rot and skin problems. If their water becomes even a little bit dirty they will develop skin problems and possibly shell rot.
Captive born adult diamondback terrapins can also be kept in fresh water. But their water does need to be kept cleaner if they are in fresh water verses if they were housed in brackish water. The salt helps prevent skin and shell problems and without it they are more susceptible to developing these problems.
Wild caught adult diamondback terrapins should be kept in brackish water (1.012 to 1.014) for the entirety of their lives. If they are kept in fresh water they will almost undoubtedly develop shell rot. Also if they are fresh wild caughts they should be housed at a higher saline level (1.017 to 1.019) for the first three months and then the level can be lowered to 1.012 to 1.014.
Wild caught juvenile diamondback terrapins can in most cases be converted to fresh water (though I prefer to keep them in brackish water). I would suggest starting them out at a higher saline level (1.017 to 1.019) for a couple of months and then start lowering the level by .002 every two weeks until the level is down to 1.008 and then you can switch them over to fresh water. Keep a very close eye out for shell rot as they will be more susceptible to developing it as the level of salt drops.
Wild caught hatchling diamondback terrapins can typically be housed in fresh water from the beginning. They can normally survive their entire life in fresh water without any problems, but as always keep an eye out for shell rot and skin problems.
I know many breeders that have second and third generation captive born diamondback terrapins in fresh water for many years without problems and I also know several people that have converted young wild caught diamondback terrapins to fresh water without any problems. Most experienced diamondback terrapin keepers are able to keep captive born terrapins in fresh water. But I know a lot of beginners and even some experienced diamondback terrapin hobbyist that have problems with keeping them in fresh water. This is why it is such an debated issue. I believe a lot of it has to do with filtration, water cleanliness and basking area. Plus some of it might have to do with each individual terrapin, some just are able to adapt better than others.
I personally keep all my hatchlings in fresh water and adults in fresh water now. I used to keep them in brackish water but as I slowly grew my breeding group with more and more captive born diamondback terrapins I made the decision in 2005 convert my group to fresh water. Overall I haven’t had too many issues. In large part it is because as of today I don't have any more wild caught diamondback terrapins. Even with a captive born group in fresh water I still have some intermittent issues with skin and shell problems but overall it has been a fairly low number.
Besides the captive born group the main reason I made the move to fresh water was because I have 9 breeding setups all plumbed together and I work with other species of turtles outside of my diamondback terrapins. If I was still working with only diamondback terrapins I would most likely keep them in water brackish at around 1.014.
In the wild there are no adult diamondback terrapins to my knowledge living in fresh water. There are several locations where diamondbacks can be found in strictly salt water miles from any brackish or fresh water. This leads me to believe that they are better off in brackish water.
The only draw back to housing diamondback terrapins in brackish water is that they need to have access to fresh water once a week.