It prob'ly should be secured..and MAY already be
All that's really neccessary is some means of making sure the tank can't slide in any direction. If your tank cannot slide at all, no matter how much or which direction your boat heels or pitches, no need to secure it...'cuz holding tanks don't bounce up and down. That can be accomplished in a number of ways...by simply wedging it tightly between bulkheads or another tank and a bulkhead...by strapping it down (plastic lawn chair webbing works great)...or by bracing the bottom corners with some quarter round epoxied to the surface the tank sits on. Regardless of which method you use, it's also a good idea to put some "non-skid shelf liner" under it--the rubber mesh stuff you can buy the roll at Walmart works just fine.
