I do 25-35% after it is cycled (I've had 20s). If it is over stocked, or just on the brim of being over stocked, 50% will be fine. The beneficial bacteria is actually in the filter, the filter cartridge and on gravel and ornaments. The water is only a small part of it :) so, when rinsing the filter and/or cartridge use old fish water and just reuse. The gravel never has to be rinsed, just stirred (works better with a gravel siphon).
Make sure your tank is really cycled. In a cycled tank ammonia should not be there (unless there is an addition which then a small or large ammonia spike will show). 8 or more weeks running with proper care is usually the time it takes to cycle.
Last edited by Sena Hansler; 01-17-2013 at 10:37 AM.