Flies? Probably a small infestation.

Water changes should do the trick.

Also, do you use salt? Some salt in the water (1 tablespoon per 10 gallons) helps a lot in fly infestations and other stuff. Just add the salt into the water gradually over several days.

E.g. 4 footer 4x2x2 (75g tank) = 7 tablespoons of salt.
Day#1 : Dilute 1 tablespoon in some water, then add into tank.
Day#2-#7 : Repeat the same.

Don't add all in one go - you might just kill ur fish!

Regards,
RazorBlade

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