Yesterday I was adding carbon to my cannister filter - I have been getting a slight odor and wanted to see if this would help. Well anyways, I hooked everything back up and came back an hour or 2 later and the cannister filter was not pumping and leaking from the side. I was freaking out, I have a 2 HOB'S but it's not enough so I took it apart and using a sink and bath tub tried to figure out what was wrong (Yea , I realize the chlorine killed all my biologicals) but anyway I tried and tried but could not get it then by chance I saw a big circular ruber piece laying by my sink where I clean my filters. The seal must have came off when I took the top off and after it was back on everything worked great. Just be careful and make sure everything is together before you leave when maintaing filter - luckily I avoided disaster but I am sure others aren't that lucky.

Also - I noticed my biologicals which were once very rough have been smoothed down - how often do I need to replace these - now I am thinking that the odor may have been because my biological media is not suitible - could this be the case?

yes, it's probably a good idea to make sure you put the filter back together properly.

I would strongly advise you not to change all biological filtration that often. It doesn't need changing, it just needs rinsing through every now and then. As soon as you change it, you are just removing all your good bacteria. If we're talking ceramic site and serial that come with Mace canister filters, that will last for ages. I'm not aware that it actually perishes, I've never noticed any of mine doing this anyway.

From now on, just clean your mechanical filtration on a regular basis, if you notice any debris getting trapped between the biological filtration, put it all in a bucket of existing tank water just to remove excess crap, and then put it back, don't go too mad when you're cleaning it.biological filtration shouldn't look shiny new.

Jelimafish has made a good point here. After cleaning your filter always check thoroughly for leaks. I then come back to the filter about an hour later to re check for leaks.

You shouldnt need to change you biologicals, just leave them be