Hi,
in my opinion, jumping fish may have severall reasons:
1. (and most common) you have a
type of fish that tends to jump, such as guppies (are they called like that in english?). Just use a cover if you want to keept them
2. your
water chemistry is terribly wrong (should almost never happen in a well maintained planted tank).
3. you give them a
scary situation by any kind and no chance to hide in safety. That's a problem for a lot of people.
The third point should be considered from a lot of different angles. While humans obviously seem like a threat for a fish as small as a few cm, there might be other reasons.
In germany, the minimum tank size for different fish is a pretty controversial topic. Some of us (and I tend to be one of them) think for example, that neons (p.a.) feel uncomfortable within small tanks (60 cm or less in length) and tend show strange behaviour, i.e. jumping. Don't know if that ever happend, it's just an example that even the pure size of the tank might cause your fish to act weird.
Another point is overstocking. This is one I really have experience with. Fish in an overstocked tank seem to feel pressed and therefore leaving the tank. This happend to me and after reducing the amount of fish, it never happend again. But the leven of overstocking has to be crazy, at least in my experience.
There could be a lot more, but just to give you some things to think about. Since the time I keep my livestock in mostly oversized tanks without overstocking to hard (I still tend to overstock every tank by a few percent without major problems), I never had any problems with jumping fish. Not even my amano shrimp are evading and they are well known for doing so.
@Pinaki Pal : Whats the problem with a covered tank and CO2?
I covered my 60 cm Tank during could seasons to stop water evaporation and have never seen any issues even with about 30 ppm CO2. The only thing is, that there need to be any holes for air exchange and at least some room above the water for air to be in. And you need to adjust the CO2 a bit, because there is less of a need. I would be really interessted in the kind of problems you expierienced, because I don't see any. But it's never a mistake to learn, most likely there is something i missed