Hi Andys101 and Clarapup
That's a nice sized tank, I only have a 180 litre......
Did you do a fishless cycle before getting the fish? By this I mean add ammonia, following the method on here.
If you didn't you will be doing a fish-in cycle and need to monitor the ammonia and nitrite levels daily. There is a sticky in the Filtration & Cycling section on how to cycle with fish.
If you have live plants, they will help but be sure to remove any dead leaves - some plants tend to lose them when newly planted.
With half a dozen harlies in that sized tank you shouldn't have any problems though. Just add more fish a few at a time.
Glad to hear the fish were gently added to the tank