I have a Fluval 405 filter that stopped gravity filling a LONG time ago with no explanation whatsoever that I can come up with. I even switched it to another tank (swapped the Rena XP3 over to the big tank and the Fluval 405 to the smaller tank).
So even that changed nothing.
It USED TO gravity fill for the first several years and just suddenly stopped.
So now I have to fill it in the sink all the way up and connect it and then it works fine. I don't get that one.... nothing changed... not the hose length or distances (as stated in the manual) geometrically. It just stopped.
It works perfectly, so I can't see the need to replace it. But I'd love to know what on earth changed. It's not missing any parts and I've even replaced some impeller-related stuff (probably unnecessarily but I had at one point lost that tiny black rubber piece that the ceramic shaft needs)
