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Anonymous
Filters DO remove large particles that go though the filter.
The reason why the gravel must be kept clean is because a lot of those "large particles" fall down into the gravel before they get sucked into the filter's intake. Think of it like a vacuum cleaner: Your vacuum sucks up dirt. But, it only sucks up the dirt right next to itself. It's not that the filter doesn't remove the bits. It's that the bits never make it into the filter.
You can improve your filter's effectiveness by getting a current going in the tank and by using a larger filter, so the current is stronger and thus there is less chance for particles to settle out (assuming that this is appropriate for your inhabitants, of course)
Anyway, to answer your other question, if your tank is set up *exactly* right then water changes are theoretically not needed. However, most hobbyist's tanks aren't set up good enough to get away with this. The real issue is that nitrates build up in the water. If your tank has a good colony of denitrifying bacteria and the right amount of live plants in it, then these will consume all the nitrates. However, without this you must remove the nitrates through water changes so they don't build up and poison your fish.
As for the gravel, again, a proper tank setup can greatly reduce your need to vacuum the gravel. Scavenging critters like some kinds of crabs, shrimp, snails, and starfish will dig around in the sand and eat waste particles. Their digging action also helps make particles waterborne, so the filter can remove them.
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