The problem with this type of pump is it easily get clogged up.. then the siphon will be compromise.
The higher pump capacity rating below.
I have to get a better pump for my new setup.
Another possible reason to the pump problem could be, after the last incident of dead fish, I run the pump 24/7 this I do to cycle the new grow beds faster.
Running the pump in this condition do not give it time to self clean. Previously I switch off the pump at night, this give time for the fishes to clean debris or foods solids sticking near the pump inlet.
I'm going to put the Timer back on... and cut the pump at night. This maybe a cheaper option.. :)