We are answering your question.. if it was temp they would huddle around the heater if there is one. The water around the filter is going to have the highest oxygen concentration which still indicates nitrite or ammonia poisoning, just because they are not dead doesnt mean they are not suffering from it. Not all fish die from high ammonia or nitrites. If your tank was fishless for a while between stockings then you have to cycle all over, if there is nothing to create ammonia there no food chain at the microscopic level, remember that nitrate is the last byproduct of that food chain ammonia is the beginning so without a beginning then all above it die.