if your tank is in a well lit room then there's no need to intentionally cover up the tank as it'll have to readjust again. all my tanks are in well lit and they are all eating fine.
it's still adjusting to it's new home, some fish takes longer than other, there's no need to try so many different things within short period of time, you'll only stress the fish out more. if you have been feeding daily without success then stop feeding for a week, don't even bother the fish during that time, you can look at it if you want to but constantly dropping food in and taking uneaten food out will only prolong the time it needed for the fish to adjust.
after no feeding for a week then drop something in, if it eats then that's great, if not then try again few days to a week later. eventually it'll start to eat. the most important advise I can give you when it comes to piranha... especially serra species is "patient"
based on the picture you posted it wasn't starved so it can easily go weeks or even months without feeding....some rhom will go on fasting from time to time, usually when they are larger
