Eggs!

The size of 5 mm. is doubtful to be a fish egg, maybe 0.5 mm.
Of course, larger fish lay larger eggs.
I have seen small tetras spawn and the eggs, although numerous, are extremely small.
I would estimate a cichlids eggs to be in the vicinity of 1 mm. or so, maybe less.
Other people with more interest in spawning fish might chip in with size estimates of fish eggs.

Bill in WI
 
What kind of fish do you have and do you have any snails? That information may be helpful in knowing if this is fish eggs or not.

Emaan
 
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