If you have plecs then courgette or the odd bit of cucumber although cucumber doesnt have a lot of nutrients in it. Peel the courgette skin off and they will eat the lot! Mine are also partial to sweet potato which I microwave for a minute to soften it up a bit, and they also like red and green sweet peppers.
For other fish try lots out - soften hard veg a bit for them. Some of my fish were partial to banana infact my gourami was called Mr Banana cos he loved them! Experiment - fish are like us they all have their own taste and they will soon you you know what they like!!