Well, anachris is not a rooting plant. It's a floating plant that can kind of attach itself with little rooting arms. As such, it's liable to always pull up and float.
If you want it planted, buying a couple of metal plant weights can help you out. Just secure the bottom with the weight and let it rest against the substrate or bury it slightly. In time, you may see some white hairy roots drop down from 6" above the gravel to the base, and will grab a light hold of things around it.