Sure, thin it out to give it more room or you can reach in there and take out the whole root ball and divide it in half to get a smaller plant. I do this all the time, otherwise they will fill in and choke the pond. Don't be reluctant to do this pond plants are very resilient. With my water lotus I get down flat on my belly reach in and cut it back underwater.

Thanks for the info Loosestrife x

You are very welcome!

its a sign of overcrowding. they wont come to any harm so either remove some now as suggested or wait until spring and lift the root stock and divide it up. I usually use an old bread knife to slice through the root stock.

Brings to mind SBG, one should always have on hand a blade sharpener. The tungsten carbide ones they sell at the hardware store will keep an old knife, scissors or garden shears as sharp as a razor and I now prefer them to the oil stones.

Yes that is so true Loosetrife2, my oh regularly sharpens all the blades on tools and the kitchen knives. sharp knives are less likely to slip and cut yourself.