Using a cycled filter

[NickD]

I have a cycled sponge filter that’s been running in my display tank and I need to transfer it over to a new tank.

obviously it’s cycled…. My question is how long do I need to keep the filter in there for the water (2 gallons) to become cycled and ready for fish. 
 

i am not keeping the sponges filter in this new tank after it’s cycled btw. The water is new tap water with just dechlorinator and quick start in it. 

Edited September 9, 2023 by NickD

[Mmiller2001]

I personally would have it in for a month. You’ll also need an ammonia source to keep the bacteria alive.

[Otto]

I keep extra HOB filters on my cycled tanks so the I have cycled filters ready for new tanks. Sometimes I put the whole filter on the new tank and sometimes I put the filter media in a new filter on the new tank. Either way I've never had problems from not waiting to add fish. I start small of course. For example when I started my 10 gallon that way, I first put in my honey gourami immediately. I tested my water every day when I did this. The tests were always perfect: 0, 0, and less than 15 ppm. Ten days after the gourami, I added 8 ember tetras. No problem. Ten more days and I added 8 kobutai and again no problem. I have also started keeping a couple of filter bags full of gravel in my plants-and-snails-only tank to add to new tanks to help with extra bacteria. 

[Huckleberry]

What I would do is squeeze that old filter out into to the new tank. Don't go all crazy with fish population at first and you should be good.