A media sock is a handy little guy. It is used in multi tank applications and or temp situations where , it is a "sock" filled with filter media.
In order for you to be able to increase the amount of beneficial bacteria that populate the tank you'll need to increase the surface area.
A second filter that is bio only and a media sock filled with pumice blocks will help a lot.
It will not reduce the number of water changes you have to do but it will help stabalize the tank a little more.
SO:
Filter 1 (original) has sponge (mechanical) and pumice (bio media)
Filter 2 is pumice only
One of those filters should flow on to the media sock.
http://www.bigalsonline.com/Fish_Fi...ia-Bags_Media-Sock_8997633_82.html?tc=default
That is a media sock... very handy little tool and I use it all the time. Especially when I need to set up a tank quick from an older existing one.