how do i top off the water in my tank?
 

a lot of evaporation, i just set up my tank a week ago. the salinity is rising and i need to add more water. i have my distilled water, should i just pour in fresh water?

If you can get your hands on a RO unit I recommend using that water, top up with fresh water never salt and an Auto top off system will help you maintain your salinity.

as of now i only have a crab and a few snails, but directly pouring water in wont stress them out will it? or when i have fish in the future

I dont have an auto top off system yet for my tank so i just use a small bucket and use a piece of air line with a knot in it to slowly syphon the fresh ro/di water into my tank. Doesnt seem to bother my fish or any of my corals doing it this way. Just a little slow is all.

No you should be good to go, as long as it isn't a drastic amount of water and your salinity/ temp levels are not taking a major hit everyone will be happy. I add top off water to my sump to ensure proper mixing before it gets to the maintank.

If it is a large top up(like at least 5% of your tank volume) just make sure that the ro water that you are adding is at the same temperature as your tank water, just use an extra heater to accomplish that. And pour it in slowly. If you know that you are going to lower your salinity by more than one point by the time you are done topping off the tank water then stop at every point of slainity lowered and wait for abit then add more and repeat the process till you achieve your top off.