Make sure you rinse the Siporax very well before placing in your aquarium.
You can use it either way as long as you have good flow through the noodles.
Siporax gives beneficial bacteria millions of tiny surfaces to colonize. It takes time but as your beneficial bacterium grow your nitrates will come down.
This is how you use Siporax. The downside is it does not come with any bacteria on it.
If you need to boost your bacteria population then I'd suggest you get some Aquaforest Life Bio Fil.
It comes pre seeded with bacteria and is ready to use immediately. I use both products actually in unison.
https://www.bulkreefsupply.com/life-bio-fil-medium-aquaforest.html
Here is a link to a supplier.
Here is a link to the Aquaforest website with the product description.
http://aquaforest.eu/en/product/life-bio-fil/