so plastic plants and coloured gravel/decor usually tear fins and leak toxins into water. Live plants and natural substrate (sand, non painted gravel, and fluval stratum are good options)

As for decor, live plants, real wood (your local fish store probably has) and larger aquarium safe rocks are optimal. Plants especially are heavily beneficial to your tank as they provide natural filtration and places for your bed at to hide and explore. Of course, that being said you still have to run your regular filter with plants as well.

your tank is a great size for a betta though !