Agree. First issue, they are likely still adjusting to a new environment. This is quite a sensitive species, we have had so many threads over the past years about new rummys dying. But having said that, other factors are important too, and one is the space and the number. This is a shoaling species and it needs a lot of them, I would never go below 12. I had 23 in my 70g for years. It is one of the most tightly shoaling tetras, and quite a sight as the larger group swim lengths, they are active. However, you need at minimum a 3-foot/90cm long tank; volume is not important (within reason obviously) but length to swim is important.