A fish tank must maintain a certain amount of "good" bacteria that break down toxins harmful to fish in order to create a flourishing environment. A bacteria bed must be established in your biological filter, which is a process called "cycling the tank." Problems usually arise in a fish tank that is poorly maintained. Two types of bacteria in the filter maintain the pH level, which is a measure of acidity and alkalinity balance to keep fish healthy.
All fish tanks contain potentially harmful bacteria. Your filter should propagate two types of bacteria necessary for the breakdown of ammonia and the removal of the toxins created by the fish's waste. These are called "nitrosomonas" and "nitrobacter". Both fish and humans have strong immune systems that protect them from the dangers bacteria can cause. However, conditions can become favourable for the bacteria if you do not regularly change the tank water or you introduce infected water. People have contracted fish tuberculosis in the form of skin infections when bacteria in tanks have passed through wounds.
A high ammonia content in your tank means you have a problem with your filter. Nitrosomonas should digest the ammonia produced from fish waste and create nitrite. This is then converted by nitrobacter into nitrate, which is harmless to fish. If your biological filter is not working and toxic ammonia becomes too prevalent, it can lead to your tank becoming dirty and can kill your fish. The dead fish left in murky water at the bottom of the tank will decay and emit a pungent smell. This and the associated spreading of bacteria could lead to hygiene and breathing issues for humans and even skin irritation.
The small bubbles that rise to the surface of the tank from the air filter spread aerosol droplets into the air that may contain biological organisms, such as bacteria, viruses or fungi. Aerosols created by strong water flow or tank aeration can carry parasites and disease to other tanks. Humans can contract these diseases by breathing in the aerosols around their fish tanks. These aerosols are only likely to affect people with weakened immune systems. An ozone generator should sterilise the aerosols by oxidiziing the contaminants to prevent them from being a threat to humans.
Regular changes of about 15 per cent of the water in the tank every few days should maintain a stable filter process in your tank. Whatever the contents of your fish tank's water it is far more likely to cause harm to your fish than it is to you. However, you should always make sure that your hands are clean as they enter the water; you wear gloves to cover any wounds or cuts and you wash your hands afterwards.