I'll note again that my experience is with Esox americanus (redfin pickerel), but I suspect that captive care will be similar except of course for the scale.
These fish are active jumpers. If you can't cover your tank, your fish will end up on the floor. I had a seven-inch fish jump more than ten vertical inches over the tank wall to its death. You need to cover the tank fairly sturdily. Even if it's in an occupied room, jumping does a lot of damage. One veteran noted that a ten-inch fish jumping will give you a scare, a 15-inch fish will have you thrashing on the floor with wet towels for a long time to get it back in the tank, and a 20-inch fish is an event you will remember for the rest of your life. A 40-incher I doubt you'd get back in the tank without considerable damage.
Floating plants make jumping likely, but will not prevent it entirely. The preferred habitat for these fish is quiet water with heavy floating or emergent vegetation where they can lie still and wait for prey to venture by.
Pike are very difficult to train to prepared foods. Although it is possible, I would not recommend keeping them unless you have the means to provide them live food throughout their life. For pickerel, guppies, mollies or convict cichlids breed fast enough, but for a northern pike you'll need something a lot bigger than that, on the order of 2-3 adult bluegill per week. You might get them to take whole fish from the supermarket, but I'm not sure I'd recommend sticking my hand in the tank. They could probably be trained to take live rodents too, although I don't know of anyone who has done that as a regular diet. They don't need to eat every day, but they will take very large food. It should go without saying that you really can't keep any other fish with them. Even another pike is very likely to end up being food at some point.
Check out nanfa.org for more info. Pike are the same species through Europe, Asia and North America. The smaller Esox species may also be available through the forum from time to time, although I don't know about the legality or logistics of shipping internationally.
Finally, please don't take offense, but realize that I don't know you at all and haven't got much information from your forum post. Captive fish are never to be released into the wild, even native fish into their native waters, because the conditions of captive care make it likely that they will spread disease into the wild population (whether or not they were kept with tropical fish - again, more information available at NANFA). If you change your mind about keeping a fish like this, it's better to kill it than release it.
Let me know if you have any other specific questions. I do love these fish.