The filter is a good place for bacteria to grow, but it seems more beneficial for filtering the water not growing clonies of bacteria. The gravel is more beneficial, and if you have a dirty filter not cleaning the water there is no point in leaving it there. Stress zyme helps aid in the growth of the bacteria. A new filter allows for the bacteria left the the filter a place to grow, and they tend to grow exponentially. And my fish always were swimming around and were very happy till I was unable to change the filter and do the water changes. They also seemed to grow quite large when I was there.
The filter is the main colonizing location for the bacteria, not the gravel like you said. The only reason for mechanical filtration is to remove debirs and other unsightlys from your tank. If your filter pad is dirty you simply wash it in tank water and put it back in thus saving the bacteria. The only product I know of that provides and aids bacteria is Bio-spira since it is refrigerated. Also, everytime you replaced your filter, they dont grow exponteially so your tank had to slowy go through a cycle again to build up a sufficient amount of bacteria. I would hope your fish were swimming around as this is what they do but is no way a sign of their "happiness." Better to look at how long they lived compared to life expectancy. I understand you are trying to help and add your two cents but most of what you have said is inaccurate, I am in no way trying to be rude. Hopefully we can continue to discuss this in a civilized manner and come to a common consensus.
Drew
Edit-After reading my thread just thought I should add you are correct in that bacteria exist in the gravel, just a small amount compared to the filter though.
First I apologize if I seem like I was rude in any way, that is not my goal. I, like you, would like a civilized forum in which people can feel free to discuss fish topics. I have not really done any research on stress zyme, but I would like to know where you received information on it not helping. As for my research, most of my research suggestions that in the perfect environment, the bacteria grow exponentially. Obviously there is no perfect environment. However, the tank seems to be a great place for bacteria to grow and it maybe more log rhythmically, where in the initial regrowth phase there would be a lag phase, but then the growth would be exponentially till it hit a stationary phase. Also, most of the places I have read say bacteria grow better under the gravel where it is undisturbed and can use the detritus for energy. As for washing your filter in the tank and placing it back in, I have never heard of someone doing this. It seems cost effective, but for me I have a basic hang on the back filter. To my knowledge that carbon in the pad does not last that long, and I change them monthly in my current fish tank. I notice a huge change in the fish when I keep the filters in there too long. Maybe you can show me where you read about this method.
very very sad 
but in going to get a bristle nose and cory :yay:
