Just checked out the feeding instructions.. 5ml per litre they say is the max.
DOSAGE:
continuously, mix in 40-50 ml. per 10 litres of water.
In the first week of the growth a small dosage suffices.
Build up from 20 ml. to 40 ml. during that period.
From the second week the maximum dosage should be applied.
Dosage PK 13-14: start with 2 ml. in the second week and build up to 10 ml. per 10 litres.
In areas with a low calcium-value (soft water) Ca 15 should be added
So it's probably over feeding causing the problem
