Sounds like you know about freshwater systems, based on your pH comment. I agree that you need to go real slow and do a lot of reading...these are totally different hobbies, with the exception of how to start a siphon
Something is wrong if a damsel dies immediately and another is stressed. What is your specific gravity, and temp? How did you add them to the tank? Do some searches here on cycling a tank, because 1) you don't need to use damsels and 2) if you are, you expect and/or want them to die, and they are left in the tank to, well, rot. In such a case, 3 damsels are usually not considered to be enough, unless you are throwing in loads of food. In which case, leave the damsels at the store and just throw in loads of food. All you are aiming for is ammonia, and there are many ways to get it.
Definitely make use of online resources and do a lot of reading.