I have to do a full clean on my Eheim 2213 once a month due to massively reduced flow. This brown slimy stuff builds up in the pipes (and the canister itself) and will close those pipes down and reduce the flow to a trickle. It's aggravating to me to do that much maintenance and it's just on a 12G. And there is nothing but RO water used on that tank so it's not something related to my tap. But I have to if I want it to work. I could get away with just cleaning pipes, but when I get that involved in gutting the system I just go ahead and do a full clean and get the gunk out of the canister as well. This particular tank though gets feed powdered food/supplements twice a week and I feed 6 days a week total (fresh veggies most of the time, other shrimp foods (Mosura, BW, Benibachi, that sort of thing) once or twice. Due to the fact I have to feed that tank almost daily I have no doubt that's why my filter needs more maintenance. I have a mini canister on a 3 gallon tank I do a full clean on about every 6 weeks or so if I see the Purigen getting funky. I only feed that tank 2-3 times a week though. When I first installed it on the tank it was so efficient I had to do a full clean in the first 10 days cause the Purigen had already gone dark brown. So I think you have to factor in feeding schedule, plant decay, and other things to get a good idea. Some folks don't do any cleaning for months on their canisters. That's what I was expecting when I decided to go for it on my 12G, just unfortunately isn't the case for me.