How much water goes past a specific rock in the tank? Only a small amount
Think about that for a minute....how is all of that water going to run through a filter if it is not moving? Your logic makes little sense to me, I am sorry.
And a UGF is still a filter and houses the majority of the bacteria in the tank.
The filter isn't holding the majority of the bacteria, it is the substrate on top of it....the filter is just dragging water through it via displacement...sort of like a siphon.
Want to test how much bacteria is on everything else? Remove your filter for a few days and watch the ammonia spike.
Don't be silly! Of course that isn't always going to happen. In a new tank, probably, in an established one, no way. People do this all of the time in a saltwater tank - including myself - and I am willing to bet I have enough water movement in my cichlid tank to do the same there as well. I am not saying that a filter is not important, but there are some very good ways of making sure that we dont have to rely on a filter as much as it seems like you are thinking. I know of at least three people with what amounts to an overstocked cichlid tank that only use a heating coil under his/her substrate as filtration. Its basically like using filter media that rarely, if ever, needs to be replaced.
If you are right, then how can the nitrogen cycle occur out of water?
When we compare live rock in a marine tank to a rock in a FW tank, we aren't comparing apples to apples so to speak
Yea it is....but maybe like comparing a red apple to a green apple. Probably the only huge difference is that we can generate a lot more water flow in a SW tank where most of the FW we keep would deal with that too well. There is just a lot more going on with live rock. For all intents and purposes, you wouldn't be completely wrong to call rock or sand in any tank "live".
Besides, the principles are so much the same that many many many people keep saltwater tanks that don't have an ounce of live rock and look exactly like a freshwater tank (hence the reason to make a comparison between fish-only and FOWLR)
I definitely agree with you about everything else though....it is best to have a filter on an aquarium, but I would add that placing something that might be carrying benificial bacteria from an established tank is at least going to give you a start and if you can get enough of it, you might be able to just clone your tank from the established one and skip a large cycling process (keyword: might).
