My outlet tube is installed aprox 15cm below the waterlevel (5 cm above the sand) in the filter. Otherwise no water will flow out of the outlet pipe. This is strange because it should be as high as the waterlevel. Any ideas why this is?
Sounds to me like the sand has too many fines and/or clay in it. I’ve had this once or twice I think, where the extra resistance is so much that the resting water level goes above the outlet level. Did you wash the sand well before putting it in the filter? What was the source of the sand? And what is the sand characteristic (D10, uniformity coefficient)? (see Filter Media section of website to know what this is).