Ok got it all set up but the pump appears to be leaking out of the circle in the bottom, could I have fitted the reservoir top incorrectly or is it a faulty pump?

I'm now worried about reports of Feser F1 gunking everything so I'm tempted to replace it with de-ionised water instead, what's the best way of cleaning out the loop do I just re-fill it with de-ionised water and pump it around for a while and then drain it again?
)
Thanks for all the advice.
When you say people left it in the loop too long how often roughly should it be replaced?