Take the feeders back and get some frozen or freeze-dried food that is made to feed your puffer.
Throwing them into the toilet and flushing them is adding to the problem.
Fish that aren't naturally found here cause problems dead or alive. Live they compete for food and live or dead have bacteria and foreign diseases that can spread to native fish wiping out hole population or species of fish
Example:
The Europeans bringing carp over to North America as a source of food in the colonial days. Carp eat trout eggs and disturb other fishes breeding beds.
GET INFORMED is the only advice.
You have made the first step by asking the questions.
Second follow the advice.
Third be prepared before taking the responsibility, i.e. books and/or other research.
Fourth have a back-up plan.
We as fish owner have a responsibility to take care of these fish, to give them a good home, with clean water, the right food, and the right conditions so they can live a good life.