Bristletooth often have elongated tail fins when small and they flatten out as they age. It's also common for them to change quite dramatically in colour, with tail colour being quite different.
If you look here https://www.reeflex.net/kategorie/5.html#katID5 especially at the pictures of ctenochaetus binotatus for a juvi picture with amazing body and tail colour and then at ctenochaetus cyanocheilus and picture 3 as it's quite close to yours and I've seen them with whiter tails which makes it a possible id.
If you look here https://www.reeflex.net/kategorie/5.html#katID5 especially at the pictures of ctenochaetus binotatus for a juvi picture with amazing body and tail colour and then at ctenochaetus cyanocheilus and picture 3 as it's quite close to yours and I've seen them with whiter tails which makes it a possible id.