Hi, I just got a new 40l tank to grow some African cichlid fry and was wondering if I could skip the cycle by putting a sponge filter from my main tank into the 40l and then wait 1 month for the main filter in the 40l to be cycled
That IS cycling your tank. Your not skipping anything. That's how many people do it. I would use water tests to determine when it's done tho, not the calendar. It may be ready in a month, maybe longer, maybe sooner. You will need to feed the filter and make sure it is fully cycled before putting fry in.
I'm not sure what you meant by " and then wait 1 month for the main filter in the 40l to be cycled", but if you add a sponge filter from an established tank, that's been in there for at least several weeks, you can add fish immediately.
I think I understand what you are asking. If you plan on removing the sponge filter once the permanent filter on the new tank is seeded then wait at the very least a full month. As long as there are fewer fish in the new tank than there are in the established tank moving a seeded sponge filter from it to the new tank will instantly cycle the new tank.
Cycling a tank simply means growing ammonia and nitrite eating bacteria. The seeded sponge filter will already have both types of bacteria on it. The new tank will take time to get established, meaning bacteria growing on all the surfaces but there should be enough bacteria to prevent any spikes in ammonia or nitrites in the new tank so add sponge filter and fish at the same time. The fish waste will keep the bacteria fed and will help it spread throughout the tank.