Tiny black Tick like bugs in tank?

[FizzyFish]

They look like terrestrial ticks but very very tiny. They are black and appear as specks without the aid of a magnifying glass. They don't even seem to move! Are these guys harmful? They're sticking to leaves, glass and even my snails shells. They are not shrimp like or long. They're almost perfectly circular.

[wolfiewill]

Can you post a picture of them?

[Laura]

limpets?