Since you're putting in the time and effort to build a backyard pond, make it one that you can enjoy for years. Pond liners are subject to the same stresses, whether they're 50 square feet or 3 acres. And while losing a few thousand gallons of water to a leak probably won't break the bank, losing all your fish to chemicals leaching from the liner might just break your heart.
A professional-grade reinforced liner is lighter, cheaper to ship, easier to install, and is less vulnerable to environmental exposure or damage from animals or dropped tools. You may even be able to skip the underlayment, a hidden cost for most unreinforced liners. Small does not equal unimportant, so don't sell yourself and your efforts short!
