why do my baby fish(fry) stick to glass with mouth, not moving?

[alexis m]

about 3 days old, in dark part of 20 gallon tank, stuck to glass by mouth. im very worried, they should be eating. the bigger fish and everything else is just fine. they are very small fresh water fish and only 5 of about 30 are doing it.
they are NOT a glass cleaning type fish

[Brandy]

they are sucking the glass to clean it