Aha... winged female aphids... would it be better if I place the pots
outside on the balcony then?

I have a feeling that the aphids came along with the plants when I
transplanted them from the garden into the pots. I have a mint plant
(very small... just a stem and four leaves) that's from a rooting and
till now it has no aphids on or around it.

I've read that planting chives, garlic or onions will also help.
Unfortunately I have no place in the pots for extra plants. Would it
make sense to chop up garlic and/or onions and place them on the soil
around the plants?

Max