Ok..here is how it works..in a nutshell.
You put together your tank..say with rock,sand,sw,etc..The rock and sand have a certain amount of bacteria in them.This bacteria needs nutrients to survive. You feed your fish,they make waste,and the bacteria breaks down the waste from amm,to nitrite,to nitrate. Once the bacteria is established,and the tank's bio-load is established,this happens rather quickly on a daily basis.Bio-load is basicly what the amount of tank inhabitants you have produce on a daily basis.This is why it is important never to add more than 1 or two fish at a time,so the bacteria can catch up with the amount of waste they produce.In a 6 gal. tank I would'nt put more than 1 fish.
A 6 gallon tank does'nt give much water volume for any of the important parameters to get out of whack.Here is what you must monitor at least until you learn at a glance if something is wrong,and even then you still need to check periodically. Specific gravity,Ammonia,Nitrite,Nitrate, Phosphates, PH, and Alkalinity.I also recommend that you get in the habit of small frequent weekly water changes.making sure that the new water you are going to use is RO/DI water,that the specific gravity,temp, and PH are the same as your tank. You must also top off daily with fresh plain RO/DI~ the water that evaporates.Stability and maturity is what is important in keeping marine life.
In the next few weeks I would go ahead and force a cycle,so you are certain that it has in fact occurred and the tank is ready to handle a fish,and some inverts. The proper clean-up crew is also important to the tank. You are really going at this the hard way..starting out with a small tank.It took me two years to get up the nerve to try a small tank.I wish you luck,and will be here to help if you need.