what type of fish?
what size tank?
filtered?
heated?
tank mates?
how long have you had it?
when did it start?
do you preform water changes, if so howmuch water do you change and how often?
more info please... there are SOOO many possibilities. could be markings, could be a disease, could be signs of stress.
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in that case, it's a gravid spot as was said by the answer bellow me. basically your fish is going to have baby fish.
your tank is too small for the fish you have in it. you could maybe have one male guppy in there happily.
I'd suggest getting at least a 5gallon tank for the guppies, especially with fry on the way. you're going to be WAY overstocked soon.
