Agreed. You should keep the softshell by itself. As far as what the animal was eating. Many insects and earthworms end up in a pond and I am sure that is what he was eating (along with the mosquito larvae).

It can take an animal quite some time to adjust to changes. Essentially -- you have moved him to a "captive" situation from what I assume was an outdoor pond at your brother's house.

If he continues to refuse food put him back where he was. There is no sense in compromising his health. Obviously he was doing alright were he was so just be prepared to release him if he continues on this path of refusing to eat.

You might try some crickets, superworms or red wigglers to entice him.

Steve