Hey Distant, That quest provides you with a Psi-Fish Spawn, this is the unit that needs to evolve. Though if it died I think the quest should auto-fail, if that didn't happen you should be able to buy another spawn and evolve that one. So what it comes down to is that the Psi-Fish Spawn is the unit that needs to evolve.
Edit: Didn't realise I was logged into my Dev Account so cleaned up the post a smidge.
Edit: Didn't realise I was logged into my Dev Account so cleaned up the post a smidge.