Red eyed tetra, Moenkhausia sanctaefilomenae
Ammonia is extremely toxic to fish, even levels below 0.2 ppm are injurious to the health of the fish if the exposure is on-going. Levels above 0.2 ppm can cause the death of fish within hours. Some fish are more susceptible than others, not just different species but there can be large differences in tolerance within the same species.
The ideal and only satisfactory level for Ammonia is nil. pH also plays a role because Ammonia becomes more toxic as the pH rises. The same thing happens with temperature because there will be more free Ammonia in the water at higher temps. This is made worse because a fishes metabolism is working faster at a high temp than a low one, so the rate at which the toxin takes effect, toxin is increased.
Ammonia occurs in the aquarium because fish excrete ammonia from their gills all the time and at an increased level after eating. It also occurs when biological waste is broken down by the action of bacteria. This ammonia is then broken down further into nitrite and then in to the relatively harmless nitrate.
If you find your aquarium has an ammonia spike for some reason then change some water immediately to bring the ammonia level down to below 0.2 ppm (mg/l). It doesn't matter how much water needs to be changed because the risk the ammonia pauses is far greater than the risk caused by changing to much water.
If there is a constant presence of ammonia, it indicates that there is a problem somewhere within the system. Possible causes of a high level of ammonia are:
The symptoms of ammonia poisoning are
Depending on the circumstances there are several things to do to treat this condition. Firstly and most importantly, the ammonia level must be reduced. This can be done in a couple of ways, first by changing a large amount of water in the aquarium making sure that the new water is de-chlorinated and is at the same temp and pH as the old water, change as much as is necessary to bring down the level to under 0.2 ppm. Ignore the rule that says you shouldn't change more than 30% of the water at once, because Ammonia is so toxic it must be reduced even if you have to change 95% of the water.
To prevent its immediate return use one of the Ammonia locking products available, such as the aptly named Ammo Lock which changes ammonia to ammonium which is a less toxic form of ammonia. This can be further helped by not feeding the fish until the levels have dropped to a safe level or even 0 ppm Then deal with the underlying cause of the Ammonia. The filter bacteria can be boosted by slightly raising the pH to above 7 (but not suddenly) and/or by adding some biologically mature filter media/substrate from another tank but ammonia is more toxic at higher pH levels so if fish are present in a tank with free ammonia do not raise the pH.
*It is important that filters are regularly cleaned and that all the biological filter media is washed in old tank water, never tap water. The Chlorine in tap water will kill the beneficial bacteria that breakdown Ammonia.
mg/l = milligrams per litre
ppm = Parts per million
mg/l and ppm are one and the same thing.
i.e. 10 ppm = 10 mg/l