Question
I'm using a water pump for an undergravel filter with 300 l/h (Eheim universal 300). If the filter is blocked because or irregularities, the pump starts to suck air because water isn't sinking fast enough through the gravel. Inserting a valve and limiting the input of the pump causes it to run smooth/without noise and without sucking air. Can this input reduction be somehow harmful for the (lifetime of the) pump?
Answer
Short answer, Yes it can.
Your product link is not English, and I did not see diagrams. Different designs can have different issues. Less water is probably better then sucking air, so your solution is likely better for the pump then without it.
The pump is designed and tested for moving a specific volume of water. When you limit this there will be strains on the pump.
Answered By - James Jenkins