When you get some time look way back in the datnoid threads and you will come to a few discussions on NTT sudden death syndrome.The info there may shed some light on your situation.
No NTT are captive bred. I don't think any dats are.Sorry to hear. Maybe something wrong internally. These are captive bred, right? Possibly bad breeding some or a few bad fish.
Yes I've kept ST too,I've had a few of all of the known dat species over the years.I haven't had the luck or the pleasure of growing a NTT to a large size but I have seen photos of large captive specimens,a fellow member here had one that was around twelve inches but he stopped posting years ago.I used to keep ST - real ST at that time. It was easy to grow them out to fifteen inches, and then they would start to fatten out a lot. But they were quite hardy. Currently I have six IT and they are similar, some currently about nine inches. I have got a NGT and an AT as well. All doing well.
I thought I was doing well with the NTT too. I will try and get some more.