A cheap hydrometer would be a good way to make sure the salanity is the same in the tank and the new water. Some folks go even farther and buy a refractometer, which are extremely accurate. You need to at least get the hydrometer.
Get at least 2 hydrometers. Sad but true they are very finicky. Glass are far more accurate than plastic not being subjest to air bubbles throwing your SG readings off.
With two you can measure one against the other, considering they are cheap (maybe $5 to $10 USD) it's well worth it.
I ended up buying a refractometer but even so still use a glass hydrometer to make sure I'm in the ballpark.
Cleaner shrimp (well most shrimp really IME) are very sensitive to osmotic changes as previous posters mentioned so any water changes need to be dead on in terms of specific gravity.
ok thanks i'll go out and buy them tomorrow morning i was thinking of waiting till i could get a refractometer but i guess it would be in my best interest(so i dont lose anymore of my precious and exspensive pets) to just get a glass hydrometer