Read the article on the nitrogen cycle it will help.
The summary is, nitrAtes are also toxic to fish, just not as much as ammonia and nitrIte are. Therefore, keeping low NO3 is one of the main reasons you do PWC occasionally. The general consensus is to keep your nitrAtes below 40ppm seems to be safe, with 20ppm or less being the desired range. The lower the better basically.
Did you monitor your water parameters closely after you put in the new fish? There's a possibility with the addition of that many fish at once that you went through a mini-cycle, where as the ammonia spiked, then the nitrites, then it all eventually converted to nitrAte. If this occurred it could have been a factor in your losses as well. Even a well-established tank is only established to the point of being able to handle the bio load it has. When you up the load like you did, it's still possible to see this so called mini-cycle.
I highly recommend you maintain the NO3 below 20 in the future. Also, when you want to add new fish in the future, you'd be better off doing it in small groups of fish at a time. I don't know that tetras are "dirty" enough to have caused this, and I understand you want to add them in "schools", it was just a thought...
The other thought I have is on the plants. I'm not a plantkeeper, so I don't have experience with them. I assume they were aquatic plants, but if you could state what they were others may be able to help you with those. If those plants aren't aquatic, and they were treated with pesticides or something, that would easily cause this.
