What Is This?

When I turned on the light in our discus tank this morning, I noticed a couple of wee specks on the glass at the back. I've taken a pic, although it's not very clear. I've seen these before in one of our community tank, but they seemed to disappear before I could get any info on what they were. They're little round things, clear with a bit of something in the middle, that slide slowly. I think I've seen antennae on them. The seem to be around in the morning, but vanish as the day progresses. Does anyone have a clue as to what they may be, and if they're something to be worried about? I'd appreciate any advice

Tank size: 125L
pH: 6.4
ammonia: 0ppm
nitrite: 0mg/L
nitrate: b/w 0-10 mg/L
kH: 0
gH: <3
tank temp: 81F

Fish Symptoms (include full description including lesion, color, location, fish behavior): No noticeable behaviour problems, except possibly a little heavy breathing with the discus (that may be me being paranoid)

Volume and Frequency of water changes: once a week/25% (last change was over the weekend)

Chemical Additives or Media in your tank: King British Safe Guard, Tetra easy balance, Fluval 4Plus filter with poly and carbon inserts

Tank inhabitants: 4 Discus (1 x 1.5", 3 x 3"), 5 black widow tetras, 3 dwarf neon rainbowfish, 3 cories (1 Sterbai, 2 Schwartzi), 1 bristlenose plec, 1 aluminum cat

Recent additions to your tank (living or decoration): the dwarf rainbows were added at the weekend, as was a piece of bogwood with live plant attached

Exposure to chemicals: none

Forgotten the name but i've had them before, harmless. I'll try and remember the name and dig out some info but I don't think they're anything to worry over.

lake limit i believe http/www.fishforums.net/content/Inverteb.../Snail-species/

That's the guy! Cheers for that...just wanted to make sure it was nothing to worry about.

No problem, always better to check