Exactly, current goes down through the gravel, through the slits in the UG filter, and up the riser tubes. The biological (cycling) activity of the filter occurs as water is directed past millions of tiny bacteria that are growing within the substrate attached to the surfaces of gravel. Powerheads greatly increase the function of UG filters. I've wondered myself about conditions under the filter and I'm more concerned with the leakage of gravel into this space which can block and alter the water flow there. I believe that most of the leakage does not occur through the filter slits but by substrate getting around the sides and under the edges esp near where the riser tubes are connected; the 2 back corners. You also have to be careful to seal all the holes in the sides of the filter before installation, if there are any. I think it's important, esp if you have finer material as substraight, to be careful not to tug and move the riser tubes after everything is set up because it can cause the back corners to lift up allowing material to get under. Once a few pieces of gravel get caught under the edges it is potentially difficult to get these edges to sit tight to the bottom again, allowing substraight to move under the edges in mass. I take a powerhead, and while pressing down on the riser tube and keeping it in a vertical position (thus maintaining a seal around the bottom edge of the filter plate), place the outlet of the powerhead into the orifice of the tube. I imagine that whatever is building up at the bottom of the tubes will then be blasted away from the area where the water leaves the UG filter and enters the tube (during normal operation). Then put the powerhead back on correctly. You can't do this with an air stone, so in that case I think it's best to not let gravel get underneath in the first place. I often punch out the screen at the bottom of the riser tubes so there can be more flow as well, but be careful not to allow any fish to get down there. I have pretty deep substraight on top of my UG filters so it's more important for me to have a powerhead to create enough suction, but if you only have an inch or so of coarse gravel with no smaller sized pieces as most plant growing mediums have the air stone is probably good enough. About suctioning the gravel: I don't believe that you can influence anything below the filter plate with that. Maybe with a very low flow situation where the air stone isn't creating enough suction there could be a buildup of organic waste, but I wouldn't worry too much about that. Keeping the gravel itself clean is the most important thing because that is where the majority of the biological activity for cycling is occuring. Maybe you could influence sitting detrius under the plates by pouring water into the tank with the intention of getting some of the pouring flow through the slits. That may move some of whatever's there closer to the riser tubes, but I doubt it's a big deal. One last thing: It's important to maximise suction with an airstone set-up by pushing the stone down as far as possible and by making the riser tube as long as possible ( close to the surface). As you can see, I have plenty of time on my hands these days. Hope I've been helpful!