there is not sufficient information. the size of the tank and frequency of water changes would help.

basically, 20% water changes weekly are a good start to maintain water quality.
add a bacteria starter like stress zyme to establish beneficial bacteria
avoid overfeeding the fish as uneaten food would make the water toxic with ammonia.

also, 12 neon's in a small tank like a 6-10G (if you have a small tank) at one go may overwhelm the tank and cause the water quality to deteriorate extremely fast. especiallly if there are no fast growing plants.