Aquatic plants are insanely easy to take cuttings from.
You cut the plant. You plant the cutting. Done.
Or, if it's got a rhizome--like anubias and java fern--you mount the cutting on something to keep the rhizome above the substrate. Or, if you have good enough root growth, tuck the roots into the substrate to hold the rhizome just on the substrate surface. While it's good to leave a few leaves on each piece of rhizome, it's not terribly unsual for a bare rhizome to happily sprout new leaves.
It's really that easy.