Reposting a picture tutorial I put together for a member demonstrating how I weigh down plants in ACF tanks to keep them from being uprooted and scattered about. I find that an oval, slightly irregularly shaped rock works best. The nocks and crannies in the rock help grip the thread and hold it in place. The thread tends to slip off a rock that is too round.
I used red thread in the tutorial so in the steps it would be clear, but normally I would use a tan or black thread so it would not stand out in the tank. You can also use fishing line. Base it on what works best with your substrate.
Step 1. Lay out plant, rock, thread, and scissors.
Step 2. Twist the thread around and around the base of the plant. This helps anchor the plant.
Step 3. Spread roots. Nestle rock up in the center. Keep in mind of a flat bottom so that the plant will stand upright in the tank.
Step 4. Wrap roots around rock.
Step 5. Secure with thread. Crisscross with thread to bind roots to rock. You can leave a few roots out if you want. Some plant species cannot have the root system fully submerged and need to have runners on top of the surface.
This is what the bottom of the rock looks like.
Feel free to post similar tips/tricks you have done!