Sponge filter

what you are refering to as ‘mature’ simply means a cycled filter with biological goodies in it.. What i mean by this is the ‘matured’ filter has benefical bacteria that will help make the water more stable and helping to decrease spiking of water parameters..though this filter should be in an esatblish tank where it can build the bacteria..and to be on the safe size let sit in the main tank for atleast a month..
it will take a good time if you put it in a bare tank and try and essentially cycle the filter..you can do this adding tiny amounts of food and maybe some fish or shrimp to build up some waste..all to feed teh bacteria..

Though maybe what you are refereing to as ‘mature’ may also mean that the filter has organisms that can feed your baby fish. either way you slice it..it’s a good idea to have a cycled filter in a bare tank.

good luck

I just plonked the sponge from the air filter into my external cannister. A few weeks later I took it out when I got eggs. Instant matured filter.

It will however, suffer a little loss of bacteria as the fry dont produce much waste. A mini cycle is almost inevitable.

I combated this by adding floating plants and doing regular water changes.