Ammonia is really dangerous to fish, even when as low as one part per million, and it’s an invisible aquarium fish killer. Fish produce ammonia via their gills as they breathe but they aren’t adapted to be able to deal with it as in nature it’s simply diluted by the vastness of the body of water they live in.
In aquaria we use filters, or more specifically the live, beneficial bacteria that dwell within filters, to lower ammonia levels naturally. Thanks to Mother Nature there are strains of bacteria, called Nitrifying bacteria, which can convert toxic ammonia first into nitrite, which is also toxic, and then into less harmful nitrate.
This is the most important part of the Nitrogen Cycle and its what enables us to keep any fish in aquariums.
So the first way to lower ammonia levels is to install an aquarium filter and run it 24 hours per day. This will become the life support system for your fish. Next is to seed it with beneficial nitrifying bacteria which can be bought in a bottle and added to the tank soon after its set up. Add a few hardy fish, add the bacteria, test the water and monitor ammonia and nitrite levels for the first few weeks and months.
Fishless cycle first
You can also do something called Fishless Cycling, where you get the water ready for the fish and mature the tank before any fish are added. Use ammonia, bacteria, and a test kit to cycle the tank and when ammonia and nitrite levels are both at zero, despite you actually adding ammonia, the tank is ready for fish.