When I visited singapore last year all the flowerhorns were housed in bare bottomed tanks with large, brightly coloured glass pebbles and gravel for them to play with. I think this kind of setup would be easier to keep well maintained. The rocks you use for the tank should not be calcareous types like limestone or marble as they can affect the pH, test this by placing some vinegar on the surface and looking for a reaction. Granite and quartz will be fine for decoration, as will large pieces of bogwood or terractota pots.
Adam
