If you bought an Oase BioMaster Thermo 250, your filter came with everything you need. The Sponges (20ppi and 30ppi) offer superior surface area for beneficial bacteria to grow on. On top of that it comes with their version of K1 media. Again, way more superior than any ceramic ring whether its from Fluval, Seachem Matrix, or BioHome. Beneficial bacteria require contact with rushing water, so all their hype about how porous it is... is just nonsense. There are plenty of papers out there and research. The Filter came with best media other than pot scrubbers, so save yourself the money.
The filter also comes with a set of Carbon Prefilters that are 45ppi. In about 3 months the carbon wears out and are useless. Just replace them with the regular blue 45ppi. If you want really great water polishing, the 60ppi is great, but you will have to be on top of cleaning them every single week, you can get away with the 45ppi for 2-3 weeks depending on how much waste your tank produces.
Also, someone above made a comment that carbon/charcoal will pull nutrients from the water. That is a myth. The pores in carbon can only capture complex molecules. Fertilizers are not complex. Now medications, those are complex molecules. The good thing is you have an Oase BioMaster and it will teach you to leave your filter alone. The only thing you need to clean is the pre-filters, the rest leave alone. I open it every 6 months to take a look and the inside is clean (the brown gunk, that is beneficial bacteria). Once a year I will will dip the sponges in a bucket of dechlorinated water to open up pores and remove any excess. The less you touch the inside, the healthier your tank will be and you will have a healthy ecosystem. On top of that, you will achieve crystal clear water, which means no need for activated carbon. If you want to use Purigen, go ahead, that will remove organics and tannins (it will not remove nitrates, ammonia, or nitrites)
The only thing I would do with that filter is buy some glass lily pipes and tube connectors. It makes it easy to remove the hoses and pipes to clean them. Other than that, enjoy your filter! I own 2 of them (250 and 650 and a FiltoSmart 100 Thermo)
