All ponds external filter 1000ef problem

[shellnchad]

1st time using an external ,set it up filled canister with tank water and primed it ready for use,switched power on and 3hrs after it started leaking,so i emptied canister and started over,this time left canister empty and primed water straight from tank itself into canister ,canister doesnt leak now ,but i seem to have an oily residue on surface of tank water,what is causing this?

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