Hello,
thank you for the feedback. The tank is 50l but I think that gonna be a bit small now. Yes I was thinking about the musk turles as I hear they do not get too big... I was thinking about undegravel filter. Is it a good idea or not realy?
Yes, unfortunately 50L is far too small for any turtle species.
Musk turtles would indeed be your best option, a good set-up would be something like 80-100L for one turtle, ideally larger. As you would expect, the dimensions of the tank are whats important, not the volume. The water will be shallow so you want a wide tank and long tank (15" minimum width, 24" minimum length).
Sorry to say an under gravel filter would be very, very poor on a turtle tank. As mentioned by Fern, they ideally shouldn't be kept on gravel in case they swallow it, but even if this weren't the case, an under gravel filter wouldn't
even nearly be powerful enough. Keeping turtles isn't like keeping fish in a planted tank, its much messier. Without a seriously oversized filtration system, the water will be full of ammonia, turtle poo and other toxins. I would recommend nothing less than a large external filter, the bigger the better.
Then there is the heat bulb + fittings and expensive UV bulb + fitting to consider (UV is preferred, but not 100% necessary for musks) .
As a side note, its certainly worthwhile checking Gumtree for people selling/giving away their turtle set-ups, give an unwanted turtle a good home. As mentioned before, these animals are a life-long commitment, but so many people fail to recognise this before impulse buying.