Most likely with live food.....next time you buy a bag of bloodworm or daphnia have a look for the other critters in the bag
I've never given them live food, just frozen blood worms. Could something like that live and hatch and grow after being frozen? Live fish, live plants, no live food.
I never used to rinse my plants when i first started. Maybe came in on a plant? It's fairly large, 'bout half an inch, so i can't imagine this is a recent thing. I only just started feeding blood worms frozen in the last few weeks when i got my frogs.
I was having another look at the thing and it sure looks spider like when it's in water. It moves as fast as a regular spider too. Double ewwwwwwwwwww FishFriend...hehehe.