coryman when setting up the breeding tank does the sponge filter need seeded in an established tank first or just put in with the fish from the start

Hi this might sound stupid, what are L numbers (iam just getting into catfish, as they say)

Cheers.

I used playpit sand and during weekly water change just run my fingers through it so it doesn't compact the tank is only 24x12x15 and the filter (fluval 2) does cause the sand to shift at the other end of tank