Parameters for fish only need to be Specific gravity (salt content) 1.022-1.026 (1.025 is ideal) temp 74-78 degrees, ammonia 0, nitrite 0, nitrates <10-20ppm, PH i keep mine at 8.2. What they mean is bacteria eats the ammonia and turns it to nitrite, then bacteria eat the nitrite and turns it into nitrates, Nothing eats nitrates so they are exported by way of eekly 10% water changes or bi-weekly 20% water changes. I personally do a 10% every other week and my levels stay as close to 0 as anyone else around. Water changes are done with saltwater that you matched to you tank parameters and temperature. Evaporated water is replaced with Freshwater (at the very minimum it needs to be treated DO NOT USE TAP WATER) I run RODI
water in mine because of corals. Saltwater aquariums are an expensive lesson when you mess up or move too fast. Adding a $50 coral/fish just to see it struggle and die in a few days isnt worth not waiting the month or so to let it cycle. Then add fish slowly. I would add maybe 1-2 then wait 2 weeks and add another or 2 more, after the tank is cycled I would not pull anything from the ocean. I would buy some good carib sea or ocean direct live sand and a box of Kent's salt mix that makes 200 gallons ( so youll have enough for water changes) its $45 on fosters and smith. Get you some good dry rock and seed it with a 10 lbs of cured live rock. You will need a filtration method (hang on the back filter or a sump whichever your budget allows, I'd get a protein skimmer that hangs on the back or in the sump if you go that route, you will need some powerheads for circulation. As far as fish anything that stays relatively small can be kept in a 30 gallon, I believe Ocellaris or percula clowns would be ok, NO TANGS before that gets kicked around, if you go with OC or Perc clowns get a Bubble tip anemone after about 4-6 months so the tank is well established. On that not the tank needs to run empty for 4-6 weeks to let the biological system level out. All of this is just my way of doing things in my tank. Everyone has their way that works for them. Get the saltwater aquariums for dummies book, It reaal did help me a lot to begin with. I'm sure there are better books out there. I dont recommend a full blown reef tank to a beginner yet especially in a 30 gal
since you will have big parameter swings if you dont stay on tp of you tank maintenance.