There's practically no beneficial bacteria in the water column. Instead, 99.9% of it, is found growing in your filter. As long as your filter is kept submerged in a bucket of old tank water, it won't affect your cycle to do an 100% water change.
If your tank has no filter, you must do 100% water changes. There is nowhere for the bacteria to grow, and without them, your tank cannot cycle. A large water change, is the only way to remove ammonia and nitrite from a non-cycled tank, because there is no bacteria to consume and convert these to a less harmful form.
When I do 100% water changes on my smaller tanks (around two gallons). I just cup my fish, take out all the decorations, and then pour all the dirty water out into a bucket. Then it's just a simple matter of roughly matching the temperature of the new water with the old, and adding your fish back in. If you like, you can also rise your substrate (gravel, marbles etc.) and decorations under some hot water, but just make sure you don't use soap on anything that goes in your tank.
Last edited by LittleBettaFish; 01-16-2011 at 03:14 PM.
If the foam is still wet, stick it back in. If your tank had been set up for 8 months, it more than likely cycled and if you took out the entire lot of filter media in one go, you will need to cycle your whole tank again.
Changing filter media is just a ploy by filter companies to get you to buy more product. I have had a goldfish tank set up for two years now (they are like poop factories) and haven't had to change the media once. Just a gentle swish in some tank water and it's as good as new.
If you ever do change out your media, take no more than a third out at a time, otherwise you risk crashing your cycle and putting your fish through a lot of stress with a fish-in cycle.
If you don't have any fish in there, or are not adding an outside source of ammonia, whatever is left of your bacterial colony will die off. You need to have a constant source of ammonia (in a cycled aquarium, this would be trace amounts that a test kit isn't capable of measuring) to feed through the nitrogen cycle.
You can do a fish-in cycle, where you maintain ammonia and nitrite levels below 0.25ppm through both large and partial water changes. In that case, a test kit would be necessary - a good quality one like API liquid and not test strips is recommended - to make sure anything like ammonia can be monitored and removed before it reaches a level harmful to your betta.