Tank maintenace

Question
I noticed a odor in my fish tank this morning and wonder what it was.  I made a water change a couple of days ago and am concerned that there may be something wronge with my filter.  I tested the the water conidition for all the regular things listed above (i.e. ph, ammonia ect.) and see that they are good. If you could give me some idea what might cause this odor I would appreciate your input, thanks.  

Answer
Hi Connie;

How long has your tank been set up?
What kind of fish do you have?
How big are they and how many?
What kind of filter system?
What kind of food and how often do you feed?
Are there any 'leftovers' anywhere in the tank after feeding?
How often do you usually make water changes and vacuum the gravel?

Let me know as soon as you can.......

At Your Service;
Chris Robbins