I'd leave it now as it will be dyeing down. if you want to split it then do it in the spring when you can see the growing points on the rhizomes. Just cut the rhizome into pieces with at least one growing point per piece. depends on how much you want in the pond.

Meanwhile, think of potential recipients. Some kinds can be prolific.

Thanks for the info
I was wondering if I should take the lily pad leaves off? x

I would wait until they start to turn yellow and brown--probably won't be long, now.

No leave them on until they turn yellow, then they will pull off easily at the rhizome

Thank You Seaburngirl :o))