Swimmers turn red if energy rises internally (8)

'swimmers' is the definition.
(type of fish, things which swim)

'turn red if energy rises internally' is the wordplay.
'turn red' becomes 'blush' (synonyms).
'energy' becomes 'e' (physics symbol as in E=mc2).
'rises' shows that the letters should be reversed in order.
'internally' means one lot of letters goes inside another.
'if'+'e'='ife'
'ife' written backwards gives 'efi'.
'blush' going around 'efi' is 'BLUEFISH'.