Frankly, it's irresponsible not to have a lid, unless your only fish are seahorses and other things that physically can't jump. All fish that are physically able to jump (which is most of them) can do so under the right circumstances, and a fish jumping out means it dies slowly. Never mind the cost to replace them, it's irresponsible not to take the basic safety precaution of a good, tight-fitting lid to protect them from a thoroughly preventable death.
Heck, even shrimp can jump. I lost a cleaner shrimp because it was the first jumping-capable thing I added to the tank, and I forgot to replace the lid. I felt terrible, poor lil guy. An inconvenience to me (fairly rare shrimp), and more than that, it died of something I could have prevented.
Sure, some tanks look better without a lid. But if you're going to have animals capable of feeling distress, you have to compromise aesthetics to prevent that distress when possible. Dying on the floor is a miserable death. Why risk an animal under your care dying that way, to avoid something that honestly doesn't look that bad? Just get a nice lid. Heck, take the lid off if you want to sit and look at the tank for awhile, just put it back on.
