Hi all,
Matt it is a fern (sporophyte). A fern spore has landed in the tank, germinated into a prothallus (gametophyte generation), which has grown and then formed archegonia and antheridia. Because the tank atmosphere is damp, the motile spores have been able to swim through a film of moisture from the antheridia and fertilised the archegonia. The zygote then grows out as the sporophyte plant we call a fern.
You can't tell which fern yet, but the ones I get most frequently are Male Fern (
Dryopteris filix-mas) and Broad-Buckler Fern (
Dryopteris dilitata), and from what I remember the young sporophyte of Broad-Buckler Fern looks very much like that.
cheers Darrel