I think it will jump start your cycle but you won't be able to skip it entirely IMO. Especially if you add fish immediately to the new 300L tank.

The bacteria in your seeded filter need to catch up to the new bioload or you'll end up with an ammonia spike. If you add a few fish at a time and monitor the water params. every day, it could work.

It would also be a good idea to use a good water conditioner like Seachem Prime to help keep the fish safe (for a day or so) when you do have ammonia or nitrIte spike in the 300L.

Good luck and welcome to the forum.

 

Thank you, really helpful. I'll gues I'll just wait then, I already popped the filter in the tank!
Ill post further updates here !

 

+1 - It's easy to think that beneficial bacteria live only in filters, but that's not the case as there is likely far more in the substrate and to a lesser extent on decor.
As mentioned, putting the established filter on the new tank will jump start the process, but you should add fish slowly or if your transferring all, monitor ammonia closely, watching for a mini cycle.

 

When starting my 20gal tank I used the filter from my 26gal. There was no cycle at all in the 20gal using this method, but I stocked carefully. Also, since there is not a big difference in water volume, bacteria in the filter were sufficient to start the cycle. You may want to stock very lightly first, since you have a lot more water volume in your new tank.

I was worried about the 26gal going into cycle since I had removed it's filter. I replaced it with a new canister, and seeded it with some filter material from my 10gal (really not that much). I checked ammonia/nitrite level every day, and never saw any measurable amount.

Keep in mind that my 20gal is planted, and my 26 gal is really heavily planted. I guess it made a difference.

 

As said by others, it helps in the cycle process, but doesn't eliminate it. Adding an established filter to a new tank can serve to quicken and reduce the cycle, but will not eliminate the cycle process. Nothing will.

For new tanks, I also always add an established filter. I also add some bacterial supplements as well as conditioned water (at different time, naturally). I still wait at least a few weeks before adding the first fish.

Adding fish is part of the cycle. It's needed. As others have said, add fish slowly. Let the tank acclimate to their bio load. It will result in healthy fish and a cleaner tank.

You can't avoid cycling a new tank. There is not magic trick to it. You can reduce the cycle time and quicken the cycle, but you can avoid it.