The 350 model for example supports a flow of 400l/h at minimum.
That´s not the whole truth. A flow of 400l/h with minimum speed (2100min-¹) is only possible at free flow - without hoses and also without filter masses (as said in the manual) - same is valid for the 1050l/h at maximum speed of 4320min-¹. You never can reach that in real environment.

In my opinion the possible minimum flow of 400l/h for constant flow is much too hight. Maybe a minimum of 250-300 should be possible to set. But anyway, that´s not a prob of GHL ....

I can't immagine 281 liter per hour as to be 1% = the minimum flow rate of the filter.
So must PLC be able to slow it down to 1% or not?
And is there may be a way to check if the Eheimcontroller is working properly?
Unfortunately the flow will decrease with time - or the pump will speed up more and more with clogging of the filter masses. That depends on programming.
I don´t know if an alarm is set able for going below a minimum flow and/or reaching a maximum speed level (as I see, pump speed is setable from 2100min-¹ up to 4320min-¹ in steps of 60min-¹) due I haven´t get my EHEIM controller yet.

But I have asked Eheim for a pump courve -let´s see if I get it.