I have the carbon and sponge in mine. The sponge always stays down but the carbon used to float and it is the right size.
Try squeezing all the filter media while still in the water to get out the air. I keep a small rock on mine even though it doesn't float anymore.
Current fish (includes all 3 running tanks): Polypterids, severums (of which I raised from eggs), gouramis, plecos (gold spot ones & a gurupa), loaches (zebras, yoyos, & a kubotai), macculocchi spotted silver dollars, an African butterfly fish, & Ctenopomas (a leopard & an ansorgii)