Just buy the 20L pond matrix like I do and you just push in a locking tab and unscrew the top to get it open. You also get a very sturdy 5.5 gal bucket this way 
I'm not to sure about the claims that it reduces Nitrates but mine never go over 10ppm and I do my water changes every 3 weeks. I do top off the tank once a week about 20 gallons though.
I'm not to sure about the claims that it reduces Nitrates but mine never go over 10ppm and I do my water changes every 3 weeks. I do top off the tank once a week about 20 gallons though.