Hey platytetrafan! Thanks for the luck, my fish need it. I tell you, everything about this cycle has been weird. My LFS
did give me seeding material (not gravel, but a bunch of muck they squeezed out of the filter of one of their tanks), which I added directly into my brand-spanking-new filters the day I added the fish. Then, when my nitrites disappeared into oblivion on day 40, I added Bio-Spira instead of Cycle (same sort of product but different bacteria and kept refrigerated for freshness). It's been two days and nothing's changed: no ammonia, no nitrite, no nitrate.
Nothing about this tank's cycle has gone as it should. I can not tell you how many articles I've read on the internet about cycling. My favorites are the ones with the little graphs that show how first the ammonia rises, rises, spikes then drops as the nitrites rise, rise, spike then drop then the nitrate rises, so predictable, just like clockwork, now you're cycled, go get some fish--oh, but if only! If I drew a graph of
my cycle, it would be flat for 20 days, then the nitrite (not the ammonia) would sloooooowly rise over a week's to a whopping 1.0; my ammonia would appear as a very small 0.25 blip then disappear, then the nitrites would sloooowly drop and disappear, and nitrates wouldn't even show up on the graph. Not exactly textbook.
I have found a very, very few articles on the internet about "silent cycling". A "silent cycle" is a cycle in which ammonia, nitrite or nitrate
never show up on
any testing over a 2 month period. It's particularly common in planted tanks cycled with small amounts of underfed fish. As the theory goes, small amounts of underfed fish create very little ammonia--just enough to establish and feed the bacteria, no more--then the bacteria quickly convert the tiny amount of nitrites to nitrates, and the plants scarf up the nitrates. Thus, the levels of ammonia, nitrite and nitrate are so low that nothing ever registers on the test strips.
My cycle hasn't been silent, but it has spoken in a very soft voice (practically a whisper!) It's been 42 days today (a full 6 weeks) and in that time, my nitrites maxed out at 1 and my ammonia made a single, one day appearance at 0.25 on day 30. I can only infer that by starting with a larger tank and by not overstocking the tank or overfeeding the fish that I have been able to avoid the extremely high ammonia and nitrite levels that many aquarists experience during cycling.
I am starting to think that my tank is cycled. This tank is chock full of mulm and muck, the dandruff-like material continues to swirl about the otherwise clear water, the fish are eating and pooping, the gravel could desperately use a good vacuuming and the filters are brown, yet I have not seen the return of either ammonia or nitrate. If I had actually "lost my cycle", both ammonia and nitrite should be skyrocketing under these conditions; they are not. And keep in mind there are three different test kits being used for testing here: the LFS's kit, my API liquid kit and Mardel 5-in-1 test strips (which, as an aside, are not crap--they mirror the results of the liquid kits exactly--they just aren't as precise).
So where is my nitrate? I know not. I have no live plants, unless you count the brown algae...could the brown algae be eating the nitrate? Possibly. It's the only explanation I've been able to find that makes any sense at all. In any event, I'm giving this tank precisely one week and 5 days longer (2 weeks after adding the Bio-Spira) and if all remains at zero, I'm calling it cycled and
moving on, nitrate or no nitrate. (The other option I've considered is throwing a slab of :spam: in the tank, quickly testing for nitrate--oh lookie, nitrate!--yanking the :spam

ut and going on with my life.
Well of course I'm joking!)
Oh yeah--the fish are doing fine. I'm a mess--they're great. In fact, one of the platies is big time preggers and is checking out spots behind driftwood and plants for a quiet spot to bear her live young. I am so not ready for platy breeding! But the idea of rinsing dead platy fry off filter cartridges makes me a little sad, so now I have to go find a never-used nylon stocking to put over the filter intake. Joy!
Thanks to all of you for your help.