"When I spoke to someone at the fish store they said I could just transfer the existing water over and let the nitrogen cycle complete in the new tank with the new water."
The nitrogen converting bacteria live on surfaces, not in the water column. The sponge-like material in your filter is important. The water in your old tank is not. Put the old filter on the new tank, making sure to keep it wet and not let it dry out during transfer.
Drip acclimation works like so: http://www.youtube.com/watch?v=ZSnJjTEjWyU
When you add food to your fish tank, the nitrogen in the proteins in that food are broken down into smaller molecules like ammonia. This ammonia generation is entirely dependent upon how much food you add, not whether or not your fish eat it. Flakes sitting uneaten on the substrate contribute to the ammonia in the water, too.
Now, once ammonia is present in the water, you have to get it out fast. If it builds up to even 2 ppm it can kill your fish. It is very, very toxic. You can do one of two things to ammonia: Convert it to something less toxic or remove it itself. The first, the conversion, is done by bacteria, whose end product is nitrate which is not toxic until 40 ppm or so. Toxicity depends on the fish species, but in general we're talking about the same nitrogen atom that was extremely toxic at even 1 or 2 ppm as ammonia being much much less toxic as nitrate. You are then able to remove the nitrate weekly with a water change, diluting the concentration and keeping it at a safe low level. The second method, removal, is done when plants eat ammonium, which ammonia is in equilibrium with. By pulling ammonium out of the water, the nature of equilibrium means that the amount of ammonia decreases as some ammonia becomes ammonium to compensate for the drop in ammonium. Then the growing plants eat more ammonium, repeat repeat repeat, and the ammonia is in the span of a few hours reduced to 0 ppm.
Now, overcrowding (it's taking me a while, but I will get to the point!). Overcrowding is when the amount of food added to the tank overwhelms your removal method and the net concentration in the water becomes greater than 0 ppm. If the tank is not overcrowded, the plants/bacteria are able to remove the ammonia at the same speed that it is generated, so the total amount present never gets above 0 ppm. But if you have too many proteins breaking down at one time, you can get ammonia in the water.
You'll have to excuse me but as a chemical engineer there is one thing hammered into my brain that I can never forget, and that is a mass balance:
Accumulation= In - Out + Generation - Consumption
In the world of fishkeeping, this translates to:
Ammonia concentration in the water = Fish food added - Mulm siphoned out + Fish dying and rotting - Plants eating ammonium
So. What are some ways to avoid overcrowding? There are two methods.
One, you can increase the volume of the tank. The way concentration works is that if you have X number of ammonia molecules, let's say 1. Now if your tank is 1 gallon, that's 1 molecule per 1 gallon, 1/1. Increasing the volume of the tank would dilute the concentration. If you have a 10 gallon tank that same 1 particle becomes 1 molecule per 10 gallons, 1/10. With a 55 gallon tank you're taking that 1 particle and diluting it in 55 gallons, 1/55. See?
The second method is to increase your conversion or removal method. For example if the bacteria/plants can eat 5 ammonia particles per hour and you add 5 per hour, the net ammonia concentration in the water is 0 ppm. Now if you add more food (because you added more fish) and all of a sudden you're adding 7 ammonia particles per hour and your bacteria/plants can still only handle 5 per hour, then the ammonia will start to accumulate and the concentration will become nonzero. You would have to either increase the number of beneficial bacteria or the number of plants. One example of a way people increase the number of beneficial bacteria is by using a filter with a larger surface area. This website, http://www.bioconlabs.com/abtqs.html
claims that waterfall hang on back filters have 200 square feet of surface area per cubic foot of filter media, and that fluidized bed filters have over 6,000 square feet of surface area per cubic foot of filter media. You can think of square feet of surface area as being analogous to houses. The more houses you have, the more bacteria-people can move in. So by switching to a fluidized bed filter, your population of beneficial bacteria can be much larger. Or you can add lots of plants. Diana Walstad wrote a very interesting book called "Ecology of the Planted Aquarium" that was all about using plants and plants alone to filter aquariums. Her tanks are wildly popular, and for good reason. The plants eat all the nitrogen waste, so you don't have to water changes nearly as frequently. For example, my own tank is very heavily planted and no matter how long I go in between water changes the nitrate never rises above 10 ppm. But that's a different story. If you want to know how to keep plants alive, just ask me, and I'll talk way too much about that, too, as I have so far with this post ::blush::
But anyway, my point is, overcrowding is really all about how much ammonia conversion/consumption is going on in your tank. And about how many volumes you have to dilute the ammonia. A larger tank is better. Using the filter from your old tank is a good idea. And no, you don't have to transfer the existing water.
Here is the latest video from my own 55 gallon tank. I use a kitty litter substrate ( http://www.thekrib.com/Plants/Fertilize ... jamie.html
) and $30 full spectrum shop lights from Home Depot (lights are cheaper when not marketed to pet owners). Video: http://www.youtube.com/watch?v=LnVXXwbIU5U
Ammonia and nitrite are constantly at 0 ppm and the nitrate never rises above 10 ppm no matter how long I go in between water changes. Check out "Ecology of the Planted Aquarium" by Diana Walstad. Plants are cool.