Again, you DO NOT need to do a *complete* cleaning!
Basically, the nitrogen cycle works like this.
You put your fish in the tank, it eats and leaves waste in the water. This waste brings high levels of ammonia into the water--ammonia is DEADLY to fish! You want NO ammonia in your tank!
In due time, bacteria will start to grow that will break down the ammonia in the water (VERY good thing!) and turning it into nitrite (which is still harmful to fish)...the good news, however, is bacteria will also break down the nitrite into a much less harmful nitrate.
This whole process tankes time (usually several weeks) where the ammonia must first build up to a fairly high level (dangerous to your fish), then the bacteria starts to grow and colonize...the ammonia levels start to go down a bit and nitrite levels start to go UP...(still hard on your fishy!)...but eventually enough bacteria will build up enough that they can adequately break down the ammonia and nitrite quickly.
All that is left is the nitrates, the byproduct of the process. These nitrates which are now left in the water are the reason you do water changes. While nitrates aren't all that bad (in low numbers) if left to build to high levels, it can be a bad thing. So by doing partial water changes (say 20%) you are in turn actually LOWERING the nitrate level by 20% as well! That is a GOOD thing!
The lady at the fish store is WRONG -- not all too uncommon. It seems like there aren't many people that understand the nitrogen cycle and how it works! Also remember, they are there to SELL fish...the more they sell,the more money they make...so if they give you *not quite correct* advice, and your fish die...you'll be back for more, and they make more money. Listen to the advice here, given by "experts" and successful enthusiasts.
The first 6 weeks or so of a freshly set up tank is the worst, most stressful, and most deadly time for your fish--also known as "new tank syndrome" where all the new fish you buy for your new tank keep dying.
In other words, for the first 6 weeks or so (until you tank is done cycling) DO NOT do a complete cleaning of the tank and DO NOT add any more fish. In fact, do NOT do a *complete* cleaning of your tank EVER, unless your fish all die of disease.
A complete cleaning of everything in the tank will kill off all the GOOD bacteria that has started to grow--a VERY bad thing. That bacteria NEEDS to build up and colonize in order to successfully break down the wastes your fish leave. No bacteria = high ammonia levels = DYING FISH. This bacteria will grow in your substrate (gravel), on your fake plants, on your decorations, and in your filter media. If you clean all those things, you remove all the bacteria, and when you put your fish back into the tank the ammonia spikes and the fish gets sick again.
Also, adding more fish while a tank is still cycling is bad. More fish in the tank = more ammonia in the tank = more dangerous ammonia levels (deadly). Wait until the cycle is over, and you have all that good bacteria to break down the ammonia...then it is safe to add more fish (slowly, not all at once)
However, in your case, I wouldn't recommend any new fish...since goldfish require larger tanks and are very messy. An overstocked tank means more stress to the fish, and more work for you to keep the water clean.
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