I've used my diatom filters for fresh and marine. I don't know what the least amount of cleaning is, but I always run them in a sink full of water and a capfull of bleach before moving it to another tank. Of course you need to get the chlorine out of it, before you use it. With a balanced biological setup, you shouldn't have to run it all the time for crystal clear water. Besides, running it all the time is a lot of work. With the tiny filtration, it needs to be cleaned a lot. Otherwise, you lose water pressure, and wear out the motor. I always prime the filters with tank water. That way I can be consistant on my water properties. I have also found that diatoms can get into parts of the unit where they should not be. I've taken the unit apart to get to these places periodically. It's a lot like getting dirt in your fishing reel. I hope this helps. I think, also, that if I used it all the time, I may miss something vital because the water looks better than it is. Remember, I've had tank water so clear that the fish look like they're suspended in space, even with a micro photo shot. And most of the time, I didn't have the diatom filter hooked up.
Gravel washing is pretty ghetto no matter how you stack it!