It doesn't sound like you have a strong fluorescent light close to the surface of your tank but for what it's worth you need to know that even grounded fluorescents create an electromagnetic field that often is very strong. It throws off measuring devices (TDS, pH meters) and if you have a small cut on your finger and put your hand in the tank you feel a weak stinging.
I used to import dwarf rasboras and I got to say the things are pretty much bulletproof unless they come poisoned by their own waste in the shipping bags. So whatever is killing them is a pretty serious issue.
First reaction should be to add activated carbon to the filter. But activated carbon itself is a funky thing. Believe it or not it releases invisible particles of itself that actually bother certain kinds of fish and shrimp. Still - I'd add the carbon if your tap water is funky. The water company often does crazy things to the tap water but you think you are doing something wrong to your tank.