WOW that is a lot of fish in a 75. I am not sure what you mean when you say "the ammonia wasn't no where near dangerous levels". In my experience ammonia is a detriment if you can read it on the test. Here is a good article on ammonia poisoning for you.
Disease Type: Environmental
Cause: Unionized Ammonia (NH3)
Description: Ammonia poisoning is one of the biggest killers of aquarium fish. It occurs most often when a tank is newly set up. However, it can also occur in an established tank when too many new fish have been added at one time, when the filter fails due to power or mechanical failure, or if bacterial colonies die off due to the use of medications or sudden change in water conditions.
# Symptoms: Fish gasp for breath at the water surface
# Purple or red gills
# Fish is lethargic
# Loss of appetite
# Fish lays at the bottom of the tank
# Red streaking on the fins or body
Ammonia poisoning can happen suddenly, or over a period of days. Initially the fish may be seen gasping at the surface for air. The gills will begin to turn red or lilac in color, and may appear to be bleeding. The fish will being to lose its appetite and become increasingly lethargic. In some cases fish may be observed laying at the bottom of the tank with clamped fins.
As the damage from the ammonia poisoning continues, the tissues will be damaged as evidenced by red streaks or bloody patches that appear on the body and fins. Internal damage is occurring to the brain, organs, and central nervous system. The fish begins to hemorrhage internally and externally, and eventually dies.
# Treatment: Lower pH below 7.0
# 25 - 50% water change
# Use chemical to neutralize ammonia
# Discontinue or reduce feeding
If the ammonia level rises above 1 ppm as measured by a standard test kit, begin treatment immediately. Lowering the pH of the water will provide immediate relief, as will a 50% water change (be sure to use water that is the same temperature as the aquarium). Several water changes within a short period of time may be required to drop the ammonia to below 1 ppm.
If the fish are in severe distress, the use of a chemical to neutralize the ammonia is recommended. Feedings should be restricted so that additional waste is reduced. In cases of very high ammonia levels, feedings should be discontinued for several days. No new fish should be added until the tank until the ammonia and nitrite levels have fallen to zero.
Because ammonia toxicity is linked to the pH, testing of both ammonia and pH levels are critical. Ammonia becomes increasingly toxic as the pH rises above 7.0. Because there are so many variables, there is no magic number to watch for. However, there are general guidelines to follow.
At a level of level of 1 ppm or 1 mg/l, fish are under stress, even if they don't appear in acute distress. Levels even lower than that can be fatal if the fish are exposed continuously for several days. For that reason it is critical to continue daily testing and treatment until the ammonia drops to zero. When ammonia is elevated for a long period, it is not unusual to lose fish even after the ammonia levels start to drop.
# Prevention: Stock new tanks slowly
# Feed sparingly and remove uneaten food
# Change water regularly
# Test water regularly to catch problems early
The key to avoiding fish death from ammonia poisoning is to avoid ammonia spikes in the first place. When starting a new tank, add only a couple of fish initially and do not add more until the tank is completely cycled. Even in an well established tank, only add a couple of new fish at a time and avoid overstocking. Feed fish small quantities of foods, and remove any food not consumed in five minutes. Clean the tank weekly, taking care to remove an dead plants or other debris. Perform a partial water change at least every other week, more often in small heavily stocked tanks. Test the water for ammonia at least twice a month to detect problems before they become serious. Anytime a fish appears to be ill, test for ammonia to rule out ammonia poisoning. If the filter stops, test for ammonia twenty-four hours later to ensure that the bacterial colonies that eliminate wastes were not affected.
