platy flashing

Bear with me; I'm pretty new to this. Set up 100 litre tropical tank just over 6 weeks ago, followed fishless cycle religiously, waited for 0 NH3, 0 NO2, got NO3 to <20, ensured stability for a week, then added 6 white skirt tetras, a week later 6 corys, a week later 2 amino shrimp; all healthy...