What stinks is in this hobby there are no guarantees. I had a buddy buy a new Marineland DD 300 that split a seam just a few weeks after he got it set up
Of course the odds are far far better in your favor if you buy a new tank but theres never a 100% guarantee on which route you go will be the safest. It's all in what you would personally risk. Me, I built a 500 gallon plywood tank but let me assure you if this had not been in my basement there is no way no how no chance I would have ever attempted it!
