yea, every aquarist will tell you patience is the most important thing. tanks need to cycle properly to get the good bacteria like fk said, and achieve steady levels within whatever range is acceptable for the specific kind tank you want to set up. salinity, ph, calcium...to be honest there is no specific "correct" level for each but ranges that are acceptable and its whatever you acclimatize your bioload to. quick fluctuations are the real problem within a tank, which is why the long cycle period is necessary. your water needs to adjust to the bacteria and levels set by the sand, live rock bio load etc. i'm by no means an experienced tank keeper but ive kept two very sucessful reef tanks for about 9 months. i mixed my water, put the sand and LR in, waited a couple of weeks, then i put a couple chromis (cheap but colorful fish $4/ea) in as suggested by my lfs just to get the tank used to some bio bacteria, same idea as fishkid's shrimp (just means theres no one way to do it). then i put my cleaning crew in (handful of snails and scarlet hermits) and waited...and waited....and waited some more for a total of a couple months. The chromis helped a lot, they're survival let me know the tank was doing O.K, and if not, then I knew to take action and i only lost a couple of bucks instead of a more expensive fish.
Checked standard levels every other day and made necessary adjustments but for the most part....let the tank be and be patient. it will work itself out given enough time.
this method worked for me both times and ill stick to it again. the next guy with have another method that works for him and so forth...but it all revolves around patience. good luck and do it up!