CichlidFish.com - Fish That Think.

If you keep species that like to dig in the gravel, but you want to keep plastic plants and are tired of re-burying them in the gravel after they float up every couple of days, then do this: Take the plants out, then after they dry use silicone sealant to cement aquarium gravel into the bases of the plants. After the sealant dries, simply toss the plants back in the tank -- when your digging Cichlid moves them around, they stay on the bottom. They will be constantly re-arranged, but at least they won't float all the time!

An addition to that one Mark:

I keep real plants in an O's tank but they stay in their pots which have a pebble siliconed to them.

I keep a number of plant pots with a pebble siliconed to them, when I decide on a new plant I transfer it across from it's pot and thread the roots through the pot and tie them to themselves under the pebble, in time I've had the roots take hold in the substrate on a couple and no amount of gardening can shift them, of the juvenile plants they still have something to cling to if they get moved (the pot and rock)