I've never heard of bananas growing under water. You must have one BIG tank to fit a bannana tree in it... I've never known bananas to grow on anything other than a banana tree.
You said you cut most parts down to 6 ft tall? Are you talking about a plant that is in your tank or outside. I dont understand?
75 gallon, fillstar xp2 with inline reactor, 12 Lb pressurised co2, 4.2 WPG-corallife 3x96 watt PC-, fluorite/silica-sand substright.