what you'll need:
1 spool of fishing line (non eco-friendly or biodegradable, it will degrade into your water over time)
3-5 shiny and/or colorful little aquarium-safe objects (certain plastics are perfect for this.
what to do:
measure out a length of fishing line that will stretch from one end of the tank to the other.
cut 3-5 bits of fishing line about 3 inches long.
tie the smaller lines securely to the long one.
tie your objects to the other end of the short lines.
drape the long line over the top lip of the tank and secure it in place with tape, glue or anything else to the OUTSIDE of the tank.
watch your betta bat and play with the shiny things!
here's a quick diagram of what it should look like: