Healthy plants, but trouble rooting

I've had several plants for the past 16 months and none of them have died with me not adding fertilizer. The only time my plants have looked bad was when I had a lot of snails in the tank who were doing a lot of harm to them.

That being said, I am NOT lucky when it comes to keeping my plants that could free-float rooted. I no longer have fry in my tank, so I don't want these plants floating, but every time I attempt to root them in the gravel, they find their way to the top within a day or two.

I'm having trouble getting my anachris to stay rooted and my inhabitants are swordtails, neons, and rasboras. I've never seen them picking at those plants, but the pineapple sword is definitely large enough to uproot them if she swims through them.

Is there any sort of fish-safe material I could use to bind the stems together and maybe weigh them down?