I just bought a 20 long yesterday. I took the filter catridge from the new filter and put it in my already established 75g. How long will it take for the bacteria to be on the catridge to where I can throw it in my 20 and it already be cycled?
And that's just to "seed" the cartridge. The 20g tank won't be fully cycled until the bateria level on the new cartridge increases to meet the load of the new aquarium. In the 75 gal tank, the cartridge is being helped out by the 75g's filter which will limit the amount of bateria on the new cartridge.
After You place the seeded filter in the new tank ,there must be an ammonia source for the newly seeded filter to feed on. Two or three fish could be added followed by intervals of a week to ten days before adding another three fish etc, etc.