That's a plant that loves to pop the surface and grow on with emersed leaves. To force it to go emersed you need to start trimming it, cutting away new youg submersed form.. This will trigger to older leaves to grow taller towards the surface. Than once it stick a leaf out of the tank it will soon wither away. But Echinodorus have a build in sensor for that. Don't cut off the first dying emersed leaves, just leave them on the plant. It will pick up that it's time to grow out emersed for leaves.
After that you will see long leaf stalks appear with a small rolled up leave.. This leaf will slowly unfold after it is above the water line and grow bigger. The plant will entirely stop growing submersed leaves and only shoot out long leaf stalks and emersed form leaves on it. After that flower stalks will soon follow with beautifull white flowers. Flowers grow in little groups and each little group of flowers forms a new plantlet on the flower stalk.
Thus isntead of thinning out and selling its runners
🙂 If it were me, i would keep them and force the big ones to grow emersed. Than you have best of both worlds.. Emersed E. cordifolius surrounded by a group of young submersed form pants.
I see you already have one leaf reaching for it.. It wants too.. And since it's an open top tank.. I guess that's what an open top tank is for.. Enjoy all that comes out the top..