[bscman]

While your tank is new, and still cycling, it is very important to keep a close eye on both your fish, as well as your water parameters.

With a fish in your tank, you are somewhat stuck between a rock and a hard spot. You don't want any ammonia in the tank, because that is harmful to your fish...however, without any ammonia there is no food for bacteria to begin to colonize. Luckily, goldfish are pretty hardy and can handle being in a little ammonia.

Here are my thoughts...
The longer your goldfish is in the same water, the more ammonia will build it. The higher the concentration, the more deadly it is. How do you get around this? Give him fresh water from time to time. Also, vacuum out the gravel from time to time...and DO NOT OVERFEED your fish. If you're using floating pellets, one or two pellets a day is plenty. Excess food in the tank creates even more ammonia.

While the tank is cycling, I'd do 10-15% water changes every 2-3 days. This will help make sure he has some fresh water to swim in, and dilute the harmful stuff to a more tolerable level.
At the same time, however, since with water changes you are also removing ammonia...it will prolong the time it takes for your tank to cycle. BUT, at least it's less harmful for your fish--so he'll be more healthy!

To do the 15% water changes, just siphon out some water from the tank into a bucket or a sink (if nearby).

Also, you'll want to vacuum out the gravel to remove the excess waste from the tank...I would do this every OTHER time you do a water change--every 5-6 days.
You'll need a gravel vacuum/siphon like the lady at the pet store mentioned. It doesn't have to be pefectly cleaned out, just get the big chunks--they'll be obvious. There should be instructions on the siphon to figure out how it works--or just ask here if you're stumped.

During the water changes/gravel vacuuming, just leave your fish in the tank. He'll be fine--after all, you're leaving MOST of the water in there.

When you are ready to add the fresh water to the tank, you'll need to treat it before you put it in. Tap water contains several different types of minerals which aren't a bad thing, however, tap water is also treated with chlorine and chloramine. These two chemicals are deadly to fish.
You'll need to go to the pet store and buy a bottle of chlorine/chloramine remover to make the water safe.
You only need to add enough treatment to treat the fresh water you're adding (so in this case, about 1.5 gallons...which is usually only a few drops-read the instructions).

There are SEVERAL different types, of which I won't comment on which one to buy. Personally, I use Jungle Lab's "Start Right" which removes chlorine, chloramine, and adds a stress coat to the water that--supposedly--helps with the slime coat on your fish.
Why do I use this? Well, I came across a few large bottles of it for a mere $0.76 each...it was TOO cheap to pass up. Works great for me, though.

After about 6 weeks, when your tank is ending the cycling phase and is becoming stable (and the good bacteria is colonized) you can relax with your water changes...and instead of doing them once every two or three days, only do them once a week.
IMO, it's better to do a 10% water change once a week than it is to do a 30% water change once every three weeks like the lady at the store told you.
Doing infrequent, but LARGE water changes, puts your fish under a lot on un-needed stress. Remember he LIVES in that water, so any big changes with the PH, TEMPERATURE, and all that other stuff, can be a big SHOCK to him!

Small water changes more frequently are MUCH less stressful, and your fish will be much happier.

Here are a few more thoughts:
Your filter is one of the key places this "good bacteria" lives. Swapping out your established filter media with a new filter can throw your tank into another mini-cycle...because you just threw out a lot of that good bacteria you NEED.
Instead of just throwing it out, rinse it out in the water from your water change! Get the big chucks of ick off the filter, and just put it back in. DO NOT rinse it off it tap water--the chlorine in the tap water will kill the bacteria. Only rinse it off with the water change water you just drained from the tank.

If you decide you MUST have a new filter insert, place the new filter in the tank for a week or so. Just hang it from the edge of the glass, or lay it on the bottom. This way it can start to colonize with some bacteria before you remove your old filter insert.
The goal here is to keep enough bacteria in the tank so you don't have to go through the cycling process again!

Also, if you decide you need to clean off your plants or decorations (due to algae build up or whatever) only clean one or two at a time...if you clean them all at once, again, you'll be removing a lot of that good bacteria. So just clean one plant with each water change...next time clean another....etc.

Most of all, just remember that the longer your fish has to swim in the same icky water...the less happy and healthy he will be. Remember, in the wild they have a LOT of fresh, free flowing water to live in...not just 10gallons in your living room...all the fresh, flowing water will dilute their wastes and they can live a happy, healthy, waste free life.
Falling behind on tank maintenance--especially when you have messy fish like goldfish, or an over stocked tank (too many fish), can be a deadly mistake...