My tank (150 litres) has been set up for a month - 4 platys in for 3 weeks, another 6 for 6 days - all seem very healthy and happy with much energy.
Came home tonite and all 10 are hiding and frightened for no apparent reason at all. Water tested - ammonia 0.6, nitrite 0.0, nitrate 7, pH 7.5...