To Salt or Not to Salt
This is the question most often asked about this species. The answer is unfortunately: yes and no. There are many examples of people successfully keeping this species in fresh water and just as many examples of people failing to be able to do so yet having failing animals turned around by exposure to brackish water. . The following advice is offered as the best combination of conditions offering the greater chances of success.
Wild caught adults need be kept in brackish water, fresh wild caught specimens must be kept in brackish water with a specific gravity of 1.018 measured with a hydrometer* (see note below) for the first six months. This can then be lowered down to 1.014 to 1.016. Without salt most wild caught animals will develop shell rot and have buoyancy problems. At salinity levels of 1.014 or above diamondback terrapins will need to be placed in fresh water at least once a week. Their body can take in brackish water and filter out the salt up to a certain level then they will stop drinking. When placed in fresh water they can drink enough water in five minutes to last them over a week. In the wild they can be seen drinking from small puddles, leaves, and even other terrapin’s shells during the rain. Rock salt is an inexpensive and commonly used salt to raise the salinity level.
Captive born hatchlings and adults normally can be kept in fresh water. If they start to develop shell rot they will need to be moved to brackish water (1.014 to 1.018).
from:
http://www.chelonia.org/articles/Malaclemyscare.htm
if you want to test the specific gravity, I think that you can take it into your vet. They have a little device that can test the specific gravity of the water. I doubt they'd charge you either. I can't think where else you can test specific gravity without buying a specific gravity refractometer
Housing
Hatchlings can be kept in a 10 gallon aquarium filled nearly to the top, they need basking platforms, and basking lights. In one corner of the environment a hardware store reflector clip light lamp should be positioned to provide artificial basking facilities. This should be positioned to provide a basking spot of 90 degrees F or so (32 degrees C) on the basking platform. The habitat should also be equipped with a full spectrum fluorescent light to provide for UVB. A UVB source is necessary for Vitamin D3 syntheses (needed in calcium metabolism). If preferred to this lighting arrangement a Mercury vapor bulb may be used that fulfills all requirements. Live or plastic aquatic plants are suggested to provide a sense of security and hiding places.
Filtration is very important to hatchlings and if the water isn’t kept very clean they will develop skin problems. Submergible filters don’t work very well for diamondback terrapins. Hang-on or canister filters work much better. The water temperature should be kept around 25 degrees C (78 F). Diamondback terrapin hatchlings normally grow very quickly and will reach 10 cm (4 inches) within the first year. They will grow out of a 10 gallon aquarium very quickly.
After the first year their growth rate drastically slows down. Females mature in about 4 to 5 years and can reach up to 22 cm (9 inches). Males mature faster in about 3 years but they only get up to 15 cm (6 inches), which makes them ideal for pets. Adult males should be housed in a minimum of 75 gallons but females need a minimum of 100 gallons. 100 gallon stock tanks, for horses, work very well for housing. They offer water depths of two feet and lots of surface area to swim around.
Crushed coral works great for a substrate. In the wild females can be found grazing on coral, this gives them extra calcium and helps keep their beak from getting over grown. Avoid having large rock in their tanks as they can nick and chip their shells leaving them susceptible to shell rot or other fungus infections.
Predator proof outdoor habitats offer many advantages over indoor accommodations and should seriously be considered as an option during warm weather. A child’s wading pool sunk into the ground in a secure enclosure makes for a serviceable outdoor habitat. Larger ponds with advanced filtration can be used to provide a spectacular outdoor home for your Diamondback.
Notes
Diamondback terrapin are not aggressive turtles but if they are over crowded, especially juveniles, they have been known to nip at each others feet. Separating the ones with such wounds and allowing their feet to heal along with moving them to a larger tank should stop the biting.
Basking is very important for adults. Females shed their scutes once a year and if they don’t bask water will get trapped under the loose scutes and cause shell rot. Males only shed once every three to four years.
