Home made bio filter media

[dan198811]

Hi all I'm setting up my 2nd 90 litre aquarium which has a trickle filter built into the hood. I refuse to pay the prices the manufacturer are asking for replacement filter and media so I have sourced new sponge from my local aquaria specialist as the filter but I'm now stuck on what to use as the bio media, again I refuse to pay £9 for 500g so I'm looking to replace it with something home made

Any ideas?