Some searching around the boards seems to get the reply "when it stops working" or "When the flow stops". I'm using a corner filter with a pond pump behind it. Trusting the manufacturer's provided chart, the pump flow rate in gph related to the head loss (feet of water) is pretty close to of flow=570-62*head. As such, the area behind my pump will run dry long before my filter flow decreases. Assuming that doesn't happen, are there any other indications that my filter needs to be cleaned? It's only been running in the current tank for a couple of months, but I'm not seeing any measurable pressure drop, which tells me it's still at very close to 100% flow.