Ok now this is weird. I just found some sort of fish-tadpole-thing in the same tank. It appears to spend most of the time in the mulm, but regularly swims along the algae covered walls or floats at the surface. I swear this is not staged.
I squeezed the sponge from my 10gal community tank, but that's never had Cyclops or planaria before. I also stored some water lettuce while I was setting up a new mini pond.
I have been seasoning my 3.5gallon for a little while now, and tonight I was stopped to look at it for a second. I'm pretty sure I saw Cyclops and planaria in the tank. I didn't put them there on purpose. Where could they have come from?