What Happens When You Use Fish to Cycle Your Tank?

When fish are added to a new aquarium, almost immediately, they release ammonia through breathing and their waste.  Ammonia is highly toxic to fish and even small quantities in the water can kill them.  In a natural environment, such as seas, lakes and rivers, it would be immediately diluted to harmless levels. However, in the confines of aquaria and ponds, levels can rapidly rise to dangerous levels unless it is constantly removed, usually by biological filtration.

New aquarist may be unaware of what happens to their fish when they are used to cycle the tank, their deaths are unknowingly, sacrificial.  When fish don't survive for more than a few weeks, if that long, they assume that it was sick or they got a bad batch from the pet store. 

The Effects on a Fish's Health
Raised levels of ammonia affect fish health in several different ways. At low levels (<0.1 mg/litre NH3)  it acts a strong irritant, especially to the gills. Prolonged exposure to sub-lethal levels can lead to skin and gill hyperplasia. Gill hyperplasia is a condition in which the secondary gill lamellae swell and thicken, restricting the water flow over the gill filaments. This can result in respiratory problems and stress and as well as creating conditions for opportunistic bacteria and parasites to proliferate. Elevated levels are a common precursor to bacterial gill disease.

Fish response to sub lethal levels are similar to those to any other form of irritation, i.e.  flashing and rubbing against solid objects. Without water testing it would be very easy to wrongly conclude the fish had a parasite problem.

At higher levels (>0.1 mg/litre NH3) even relatively short exposures can lead to skin, eye, and gills damage. Elevated levels can also lead to ammonia poisoning by suppressing normal ammonia excrement from the gills. If fish are unable to excrete this metabolic waste product there is a rise in blood-ammonia levels resulting in damage to internal organs.

The fish response to toxic levels would be lethargy, loss of appetite, laying on the pond bottom with clamped fins, or gasping at the water surface if the gills have been affected. Because this response is similar to the response to poor water quality, parasite infestations and other diseases, it is important that a proper investigation is made to establish the real cause before administering any treatments that may exacerbate the problem.

However, ammonia is necessary to start the nitrogen cycle, this is the first step in establishing a biological filter. So many people are told to use fish to start this process, but what they don't tell you is that this process is harmful to fish.  You really end up sacrificing your "starter fish" to get the nitrogen cycle started when you don't have to.

Originally printed @ http://www.fishdoc.co.uk