I would put it in the media basket. Just remove some the media to make room for the purigen bag.

 

I just received a bag of Purigen two weeks ago and am so far very impressed. I have a 2213 classic with a media basket. I took out the top-most floss pad and put the rectangular bag in it's place. I don't know the configuration of the ecco but if the filter works like the classic the placement would be the same. Water comes in at the bottom and is filtered from coarse to fine as it reaches the top and gets pumped back to the aquarium. You want the bag of purigen at the end of this process, at the stage just before the water goes back to the aquarium. Put it in and enjoy the clear water, I bought two!

 

+1 Lancedduffy. Best place to put it is at the end of the filtration cycle so it doesnt get packed with fish poo.

 

i put it on the top tray on the top of substrat pro right before the water goes back out.

 

I put it in the last basket before the end of the cycle also, and its turned my water crystal clear finally. This stuff really does work apparently.

 

Agreed- last stage so it doesn't get clogged, regardless of what filter you're using. Purigen is easily recharged with bleach solution once it turns brown- works great for tannins and general cloudiness. I've never had luck with activated charcoal but this stuff is always worth the price.