salinity

[deep6er]

For some reason i am having trouble getting my salinity up. Its a new tank (33g) it has been runing for about 2 months. i just put 3 green chromis in the tank about a week ago. i did a 25% water change on monday. Filled it back up with water at the right salinity. But for some reason the salinity is low ( it's at 26ppt) 1 chromis has died today. I keep adding disolved salt but it goes up just a bit. I don't want to add to much.
What am i doing wrong? Does temp have a big part in this?
Sorry for the long post, but i just do not know what i am doing wrong.