My guess would be with the length of run, you are not getting enough voltage at the furtherest end of your run.
You can run a pump like that on a 12V circuit as long as it is run like a ring circuit, with the lighting on it too.
Most ELV pumps are of the Shaded Pole type, wound to a certain voltage and they only rotate one way, and they only work on AC!.
Just make sure that your wires are large enough to carry the current.
Please if you have any more questions, fire away, mate!!.
Let's face it, these days if you're not young, you're old - Red Green