I agree with blue but are you also checking the water parameters? That seems a lot of fish to have added so quickly. Have you any test kits? When starting a new tank it helps to check the pH, ammonia, nitrate and nitrite levels so you can tell if the tank is cycling properlyl. It will also help you decide when to do partial water changes.
Leaving the tank empty for a week would have done nothing except allow the gravel dust to settle and for you to check the consistency of your heaterstat temperature (which is a good thing to do). It will have not started cycling until you added some fish.
A 280L corner unit sounds good. Do you have any pics?