I've had an under gravel filter in my 75g for a few months and all it ended up doing was storing poop and nasties and eventually causing problems in my tank. I found the usual air stone down the tube just didn't pull the water through enough. So I took the little top bit off and put the end of the filter siphon hose down the tube instead. As soon as I turned the filter back on I saw load and loads of poop just get sucked straight up and now two days later my water is more crystal clear than ever. It's a 3ft UG filter so doesn't cover all my tank, but seems to be enough to make a massive difference. I've included pics. Not sure if this has been done before but hope it helps someone out there