Yes, fishies can eat cucumber although its mainly given to tropicals and also used to catch stray snails that might have come of your plants
As for veggies you can give your fishies well they love peas and also heard of people feeding them carrots aswell and know one lady who feeds hers broccoli florets squashed up

In all instances cook them (apart from the cucumber obviously

or defrost them and skin them if using pea's) and then squash them between your fingers into the tank and then remove any thats uneaten
