Jan18/16
Pleased to report that ammonia and nitrite are still zero after Saturday's cleaning. So I know now how much to clean the drops. While phosphate remains at zero, nitrate remains stubbornly at 5.0ppm. I changed 2 gallons of water yesterday but don't intend to change any more for a while unless nitrates (and/or ammonia) spike again. On the premise that something is generating nitrates and the most likely suspect is the attached Eheim, I took the Eheim offline after today's water tests. Because the algae in the drop is maintaining phosphates at zero, I conclude that the algae is growing normally and can eat whatever ammonia gets generated by the fish, snails and detritus decay.
My original purpose of adding the Eheim to the tank, to eliminate floating debris, has been pretty much replaced by the fish. They eat all the stray algae and everything else settles out where I can syphon it from the bottom every morning.