Over the last few weeks, my pool gradually got greener and greener (algae).  I thought it was unusual because I had not had an algae problem since I installed a Pentair Whisperflo Dual Speed (Up Rate 2 HP Pump).  Moreover, it was also unusual because it has been extremely cold and I typically don't have a problem during this type of weather.  Interestingly, I would backwash the pool and the backwashed water would be a very, very light green.  I would have expected a dark green backwash.  Over the last few days, the water flow back into the pool has gradually diminished to the point that it is now down to a trickle.  If I backwash the pool the water flow is about half the usual rate.  I have opened up the sand filter and there does not appear to be a problem.  The basket in the pump is clean.  I notice (through the see-through glass) that the water flow into the basket doesn't appear to be at full flow.  So at this point, I think either the pump has gradually gone bad or I have some sort of clog.  I don't think it is a clog because the water flow is of sufficient strength that if there was a clog it would get cleared by the water flow.  So that leaves the pump.  Should I return it?  You can look at my account and see that I purchased the pump around mid-summer.