the intake tube from the left side of the tank goes straight across for about 1.5 m after it goes down and the right side intake tube just goes straight down to the filter. the output tube from the filter goes straight up then across to the spray bar in the tank. all tubing is above the filter height. even when this filter was on my other tank with shorter tubes the flow rate was the same as it is now.
The filter is packed with 1 complete Eheim 2080 media set which is if i remember correctly 4 litres of efhimech and 8 litres of substrat pro. This is what i originally thought may be the problem, so i tried taking the media out for a minute and running it empty but the flow rate still stays more or less the same.
Finally, the top of the tank is approx 4ft off the ground, about 2ft higher than the top of the filter, although the 2180 should work fine up to 6ft from the base of the filter according to the manual..
any ideas what else it could be? need to get filtration properly sorted before stocking the tank any more