Tinfoil barbs have an adult size of about 14 inches and one of the fastest growth rates of any fish ive come across, a 3" fish can easily be a 12" fish within a year.
the point people are trying to make is that your tank isnt big enough for one of these when full size. if you cannot get them a bigger tank they will reach a size thats going to be hard to rehome. as for food nahh they dont eat that much. they make great tank mates with oscars as they will work as a clean up crew living happily on the debris
one poster advised a 150 gallon tank and id have to agree with that. didnt you research these fish before buying them?
I had 4 of these years ago, they were fine until they reached about 8 inches but then became the terrors of my tank. Their appetites became so huge that they would eat almost every scrap of food before anything else could get anything to eat, they would also chase and bully off any fish that came up in the water level to try and compete, there was no food they wouldn't eat ranging from lettuce and courgette for my plecs through to lance fish and mussels for my predatory catfish and anything else inbetween. They final straw came when one of them litterally ripped the gill cover off of my lima shovelnose in an attempt to get to a mussel it had in its mouth, all 4 tinfoils were bagged boxed and returned to the store that day.
I would only recomend these for public aquaria or very large species tanks.