Prata can position on rock or sandbed as long as sufficient area for them to spread and dont accumulate detritus. Low flow to medium flow is preferred. I dont feed my prata and they are doing well for the past one year just depending on indirect light. Observe the colours and opening, if they do not open well or if their colours bleach, change a position The flow or the light might be too strong for its liking. Dose just a little iodine can help in "repair" and colours recovery.