The typical registered Sectional Title rules state that the Trustees must approve of all external improvements. However, if they object without any valid reason, you can approach the Court for its approval/permission, and if the Court approves, the Body Corporate has to pay the legal fees. Therefore, they will only object if they really have a valid reason.
If your pond does not affect the aesthetics of the complex negatively, the Court will grant permission.
Normally the Trustees have no objection, provided that you adhere to all building regulations.