I have lost SAE by not having a top but no other fish recently. Personally, I don't like the fact that it blocks light (especially if it's dirty). But the main thing is that it traps heat. Because of my lights, it's hard to keep my tank below 82 degrees as it is. Someone once told me that floating plants can prevent jumping, but that would block light too. If you do a search, I think I remember a thread about various contraptions to reduce fish jumping - like optical illusion kind of things (could be remembering wrong though). One negative of not having a top, besides fish jumping, is that water evaporates much more quickly. That's fine for me b/c it reminds me to do that weekly water change.