I live in East Texas and I found a Musk turtle in the Road about 5 months ago. He is about 4.5" long & I have a 60 gallon tank with Koi so I put him in it, with basking spot, a variety of food, plenty of places to hide, did everything possible to give him what I thought was a safe and natural feeling home. He escapes his tank constantly, he is an excellent climber. My questions are is he escaping because he is unhappy and wants to be returned to the wild or is this just normal behavior? & After being a pet for five months, if I release him will he be able to fend for himself still?
In future, it's best to leave wild turtles alone, or move them to a safe area. They don't do as well in captivity as captive-bred turtles, and the wild populations have been severely affected by collection and need the adults to ensure their future survival. At this point, however, you can't release him. He would be able to take care of himself, but since he's been in a tank with the koi there is a risk of introducing foreign pathogens to the wild population--this is why most states have laws against releasing pet reptiles.
If turtles can see a way out, they'll attempt to escape, and once they've figured out an escape route, they'll keep going back to that same area. It's natural behavior for them, and your turtle of course is used to having far more room than he does now. If you can get a bigger tank, it might help somewhat. More room is always better. Also check your temperatures to make sure the basking spot isn't too hot--it should be about 90 degrees as measured on the basking platform right under the lamp. Do you have a good UVB bulb? That's critically important for a captive turtle, and many of them aren't that great (ZooMed Powersun is a good one). Make sure that your filter is really excellent, too. Most problems with skin and shell are due to poor water quality, and with the combination of fish and turtle, you want to make sure the water is very clean. The filter should be rated for at least double the tank's capacity, and in your case three times wouldn't be a bad idea.
If you can provide live food at times, as well as plants for grazing, that will allow for some natural behaviors. He will probably settle down somewhat in time, too. Good luck with him, and let me know if you have any more questions.