water temp and brackish water

I don’t have a water heater in my terrapins tank. The temp is usually around 70. Is this too low? I didn’t know if I should get a water heater or if the temp was ok. I was also trying to find people who keep their terrapins in brackish water. Please let me know if u do! I just shocked my hatchling and am worried about when to increase the salinity levels and whether or not to keep him in freshwater every so often.