This is getting weird – I sent a water sample to my local (1 hour away) saltwater aquarium store. He phoned the results:
Kh 15 – which is not that wierd since this is the same as the tap water (lots of calcium)
Nitrates > 25 mg/l !!
Phosphates 0,3-0,5 mg/l !!!! which is really weird since I measured around 0,3 before the water change
My own test-kit (Red Sea algae control test kit) measured nitrates at around 2 mg/l and phosphates almost 0 – so now I am confused, since I just changed 600 liters out of a total of around 800 in the whole system. I ordered a new test-kit for phospates – hope that will give me some direction off where my aquarium is headed.
Today I built my own design of a portable deep-sand bed, using a box for refrigerators from Ikea, some pvc pipping, parts from a gravel bed filter and a piece of nonwovens (textile for preventing the sand from running out the drain in the bottom). I hope this will help reduce nitrates by denitrification.
Here a schematic of the whole system as it runs now.