It is best once the plant acclimates and gets adjusted and grows some first. Once it has grown. Cut the middle of the plant, say 4 or so inches from the gravel.
that is a fast growing weed like plant, pretty though. just make sure you cut from the bottom not the top, an then replant the top parts. this plant does good floating or in substrate. when left floating they make and awsome root maze for you smaller fish to hide and play. but you can cut it at any time its a fast growing well adapting plant.
Wait...so when you're trimming your anacharis, you cut off the bottom and throw it away? I had been trimming the tops off of mine as they got too tall. Interesting!