mysterious creature in my coldwater tank

I bought a BiOrb 60 litre 6 weeks ago. Left it for a week on its own then added a goldfish and a staghorn snail. After a week or so there were jelly-like clusters appearing around the tank which I later found out to be snail eggs. there were 9 in total so I removed most of them and left two, an older one and a very new batch. 3 days ago I added my second goldfish to the tank and he settled in fine but kept disturbing the snail eggs. I can down this morning to find the older batch of eggs which was full of black spots had vanished. I can only assume the eggs hatched. I've seen 3 tiny black dots on plants so assume the other ones have either been eaten by my fish or are around somewhere. I noticed at the same time the eggs vanished that a mysterious creature appeared. It's flat and about 2mm in diameter. Clear with black lines inside it, two black dots at the front which I assume to be eyes and paddles at the back to propel itself around with. I've never seen anything like it. I spent a lot of time watching the tank after I added the new fish to make sure everything was ok and haven't niticed anything in the 3 daysI've had him so can only assume its something to do with the snail eggs. My two fish have tried eating him on the odd occasion bu keep spitting him out again. I don't have a spare tank but will get one if it will help keep him alive but I don't have a clue what he may be. Anyone got any suggestions?

unfortunately its far too small and fast to take pictures. it won't stay still for long enough..

I've found out what my mysterious creature was... it was an argulus (fish louse in laymans terms). Got rid of it and informed the pet shop to treat for it too. Cna't believe I thought it looked quite sweet!