by big, it needs to be the gigantic kind that takes bigger ****s than your fowerhorn does. I'd say the pleco should be double the size and remain so throughout if you want to try and keep them together. Hiding don't work, flowerhorns will move stuff just to get at it and at the same time damage themselves. It's not impossible, but I tend to stay away from them, they seem to dirty the tank as much as they clean it.