Easiest choice is to go with Fluval filter media as it's made for them.
I have some Fluval biomax and I got some Marineland ceramic media because it was on sale on Amazon.
I really want the Bio as its for my ray tank mechanical is easily achieved with the sponges thanks for the replies I have them full of eheim substrate pro right now but wanted to see what others are using
I filled mine all up with Seachem Pond Matrix. I've read a few articles saying that Matrix has more surface area than Eheim Substrate Pro and Fluval media rings. Plus, it's cheaper than those other two.