@davgriggs do you have any root tabs in the substrate? such as Osmocote+? how long you had these fish for? where did you buy them from? i myself would loose 50% fish in the first few week to month if i bought them from petsmart. i wont comment on high NO3 killing fish but i can tell you one thing, when i dosed high level of nutrients, fish and shrimp would randomly die and most of them would no pass over 6 month to a year. other possible reason in your case could be very high co2 levels, i never seen neon or shrimp going on top of the surface when co2 was high, instead they look lazy or hiding all the time. if you ever have a concern about High No3 or you ever change your mind, you can simply use NH4/Urea based nitrogen, this will give you much better plant growth and keep the NO3 low as well.