I know this is an old post but I am a new member here but I am also a very experienced fresh water turtle keeper and I thought I would just post my thoughts.

The common rule of thumb is to never put anything with your turtle if you want it to survive, especially a ELN as they are good hunters and will eventually get hold of your BN.

The best thing to have in your turtle tank with your turtles is only the feeder fish you want them to eat (the cheap feeder fish from LPS)

I will admit, I had a couple of my BN in with my short neck turtle (I have many many turtles) and they lived happily this way for about a year, then all of a sudden I found the BN fighting for life with it's tail end completely missing.
I managed to save him and it has regrown but in a odd way and this is the reason for me joining up here today, cause he has just had his first spawn and I knew nothing about what to do cause my BN were mainly just for helping keep my tanks clean, not for breeding purposes.
So it's just not worth it if you want your BN to survive, cause nature is nature and a turtle needs to eat too:)

Also, due to the size of your lil turtle and to avoid any health issues now or in the future, something a lot of people are not aware of with captive bred turtles, is with any feeder fish you put in the tank, please ensure they are no bigger than the turtles head otherwise they can get intimidated easily and this stress causes health issues quickly.
Also make sure you have plenty of fish in the tank for him to catch, especially if it's a big tank.
For a 3ft tank, I suggest at least 150 feeders.

I hope this has been helpful somewhat, your welcome to PM me if you want to know anything else turtle related.