Well the Amazon (and Petco) ads are wrong as it is not a Marineland Penguin unless there were changes to the model name in the last couple years.
There should be some notches on the intake pipe as well as the intake strainer that prevent the sections from rotating. If the nubs that engage the notch are missing or worn down, that would explain why it's rotating.
You could do as F FJB suggested and use a wrap or two of Teflon tape to keep it in the proper position, just be sure no excess tape is able to be removed by your flowerhorn.
There should be some notches on the intake pipe as well as the intake strainer that prevent the sections from rotating. If the nubs that engage the notch are missing or worn down, that would explain why it's rotating.
You could do as F FJB suggested and use a wrap or two of Teflon tape to keep it in the proper position, just be sure no excess tape is able to be removed by your flowerhorn.
