Flies / bitey git things appeared on fish tank

[popegregory]

BUT I don’t think its the tank as the tank has been here years with no problem and the flies only appeared on it when the guinea pigs got here about six weeks ago and lived inside nearby because of the cold. Any suggestions what might have happened - we’re thinking it’s because of the hay as it’s a bit coincidental - or even better what to do? We’re thinking about removing fish for a bit and getting tank out of house as well as getting Scampy and Snowy outside too when the weather warms up too and hoping this’ll sort it.