Hey, welcome to the amazing world of snakeheads, then!
I'll share what I know until DC adds everything I've missed!
Plants, plants, more plants, lots of plants, with a bit of plants on top.
Channas feel secure with plants and overall cover, display better behaviour, and in your case, given enough space, might even not kill eachother off though that's always a gamble, and is most usually the exception rather than the norm.
The usual way of having channas is to buy 5-6 and hope for a pair to form, then remove every other one, but if I understood right, they're not quite common where you live, so that might not be possible. Either way, I've had bleheris live in a group of 6 in a big enough tank, with so much plants they squeezed themselves out of it at food time. ;D Ornatipinnis tend to be a bit more on the aggressive side if I remember correctly, so you may end up with just one...
As for feeding habits, best is a diet of insects (You'll enjoy handfeeding mealworms, I can promise you that, they go crazy over them), but they will usually eat everything hitting the water, they're probably the easiest kind of fish to get used to pellets. Nevertheless, insects are still to be prefered if available.
Other than that, I guess dim lighting is a plus, but provided enough plants, that's going to be the only way to have it anyway.
Oh yes! They are amazing escape artists, make sure your tank is covered, and doesn't have any openning, including corner holes you'd deem too small for them to squeeze in, they will, eventually, end up on the floor otherwise (Found a dry channa on the ground, and god knows how, it survived once put back into water!).
And I guess I'll never finish this post... They are air breathers, which means they do need acces to air (if you got duckweed as a surface cover of plants, don't worry, they'll come up and breathe through it). If the surface is 1 or 2 centimeters underneath the cover, that's plenty enough, but they do require it.
I hope you enjoy your channas, they're amazing.
Edit: I knew I forgot something! They're sub-tropical fishes, they need temp drops in winter, and warmer temps in summer. What I know of denmark makes me believe you may need to regulate that through heaters, since your winters are cold to say the least, the norm is not to use a heater at all as they will only thrive with temps fluctuating between 10-12°C and 20-22°C as seasons come to pass.
It sounds complicated, but it is much easier than it seems once you got the hang of it.
