Oh dear, I had a feeling you might have to do that. Sorry for your fishie loss. 
Had a look through your reply to those questions - can't honestly see anything wrong with all of that. If none of your other fish are showing any signs of illness then I would just monitor the situation over the next few days to make sure they are all looking healthy and happy.
I know you said that they tend to hide a lot but maybe these particular fish prefer to come out when the lights are off? I've not kept rummynose tetras so just guessing on this. Could you perhaps do a bit of reading up into their natural habitat and how many hours of sunlight they would normally be exposed to in the wild? Maybe these fish like to hide in the shade anyway?
The only other thing that I can think of as to why you've suffered these recent problems is that when you added the new fish you added too many in one go. I think the general rule of thumb is to add only 2-3 new fish, allow sufficient time for both types of filter bacterias to build up to an increased level (a week or two at least) to deal with the extra waste produced and once that has stabilised you can then add a few more, and so on until your tank is adequately stocked.
Adding too many in one go can cause a spike of ammonia/nitrite in your tank as there won't be sufficient bacterias in the filter to cope with the sudden increase - and even a tiny spike can be enough to affect some fish, especially if it's already stressed out from the move from the LFS.
Well, let's hope things settle down in your tank now.
All the best - Athena
Had a look through your reply to those questions - can't honestly see anything wrong with all of that. If none of your other fish are showing any signs of illness then I would just monitor the situation over the next few days to make sure they are all looking healthy and happy.
I know you said that they tend to hide a lot but maybe these particular fish prefer to come out when the lights are off? I've not kept rummynose tetras so just guessing on this. Could you perhaps do a bit of reading up into their natural habitat and how many hours of sunlight they would normally be exposed to in the wild? Maybe these fish like to hide in the shade anyway?
The only other thing that I can think of as to why you've suffered these recent problems is that when you added the new fish you added too many in one go. I think the general rule of thumb is to add only 2-3 new fish, allow sufficient time for both types of filter bacterias to build up to an increased level (a week or two at least) to deal with the extra waste produced and once that has stabilised you can then add a few more, and so on until your tank is adequately stocked.
Adding too many in one go can cause a spike of ammonia/nitrite in your tank as there won't be sufficient bacterias in the filter to cope with the sudden increase - and even a tiny spike can be enough to affect some fish, especially if it's already stressed out from the move from the LFS.
Well, let's hope things settle down in your tank now.
All the best - Athena
