[lohachata]

first mistake......listening to a petco employee.....
second mistake.......adding the water treatment....
third mistake...no filtration......
common mistakes for many.......you probaly would have been better off using a sponge filter....which you can still do..
a single betta in a 5 gallon tank will create a more gentle cycle..
hmmmm...what do you mean by "toxic" ??
petco plants.....OMG.....most of the plants they sell are not even aquatic plants..and some may even give off toxins that can harm aquatic life..

get all of your info on the stuff and figure out what they are for..go/no go for aquariums...
hopefully you will have better luck with your next betta...it is hard when you lose your pets..