I’ve been feeding Tetra flakes for 40 years, and I have to admit I didn’t even know it came in buckets this size until I saw a sale on Chewy… now my wife is wondering what my fish plans are for this year…
Now that's how you buy fish food! I'm way too cheap to buy small quantities when I can get the large economy size.
I would suggest subdividing that into a number of smaller portions (maybe the amount you would use in a few months' time) and then store in the freezer. Open and use one at a time, preserves freshness. If you have a kitchen vacuum sealer, that would be perfect.
I'm pretty sure that the best-before date is assuming that the package is unopened. Once you open that monstrous tub of fishfood, you can't expect the contents to remain fresh and optimally nutritious for more than a couple/three months unless you take steps to repackage and freeze.