This is what I use.
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I don't know if this is an urban legend regarding fish, but I heard/read years ago about a fish (Pleco?) getting caught in the ring the rind left when an entire circle slice was put in the tank. This killed the fish.
So, since then all cucumbers, zucchini, yellow squash, etc. that goes in my tank are slices cut in half. Because I figure it's plausable it could happen - even if it isn't true - and so I'd rather err on the side of being cautious.