I recently discovered the Hygrophila polysperma is listed as a federal noxious weed, making it illegal to transport without a license. It didn't occur to me to check state laws. Turns out, it appears to be illegal to possess the plant in Oklahoma. So, sadly, I decided to get rid of it.
In three of my tanks that isn't a huge deal - while it was in all of my tanks, it wasn't a major component of the flora. In the plant tank, on the other hand, it was clearly the dominant plant. When I established the tank I took a lot of small bits and planted them with the hope of getting good stems to establish elsewhere. So not only was there a lot of biomass, there were an awful lot of stems of the stuff. Removing them is not only traumatic to me, it's also hard on the fish. After living life well-hidden, they are not out in the open. And I have to start over on my planting ideas.
The other challenge, of course, is disposing of the stuff. Two options are not available - sending it down the drain or throwing it in the garbage. Luckily, it has never flowered, so I don't have the worry of killing seeds. Still, just to be on the safe side, I decided to microwave the plant material for five minutes, and then let it dry completely.I can't see how it would survive that. Now the next challenge - getting the "plant tank" back to the point where it deserves that name.
Update: The challenge, I think, isn't killing the plant, it's finding all the bits. The very characteristics that make Hygrophila polysperma a problem plant also make it hard to eradicate - it can sprout from small pieces of stems floating somewhere in the aquarium (or body of water).