mine were mexican turbos... had a total of 5. they all eventually died out in the first 2 years. 78-79F constant temps, 1.026 SG etc. didn't die due to a lack of algae to eat. got a new turbo about 6 months ago, so far he is happily munching away as I type...
I believe, as was mentioned in the other thread, that they all died due to the warmer temps than they are able to endure for a long period of time. nothing else could I find out why my bulldozers were dying one by one.
just a long shot. have you tested for copper in your DT? probably not an issue, but I would check to make sure somehow copper wasn't introduced. that would be the only other thought to why, other than some disease/parasite.