I have perfect parameters, why are my fish dying?

The following is a common statement on online forums and fish groups

"I am measuring 0ppm ammonia, 0ppm nitrites and low levels of nitrates, have a pH of 7.0. My parameters are perfect, but I am regularly losing fish, why?"

Measurable parameters do not fully dictate water quality

While the absence of measurable ammonia or nitrites can be a good indicator that the tank is cycling properly - that ammonia and nitrites are being broken down and oxidised by microbial action - it is by no means the only measure of water quality.

Water can measure 0ppm ammonia and 0ppm nitrites while still carrying a high load of pathogenic bacteria, for example, or be contaminated with heavy metals or other forms of organic waste.

Take, for example, fish food pellets left uneaten at the bottom of the aquarium due to overfeeding. Without a functioning filter, we will find that it only takes a tiny amount of waste food to cause the water to become cloudy as the microbial population multiplies rapidly on the available food. Depending on the exact microbial mix in the tank, this could lead to an increase in pathogenic bacteria that, when paired with stressed fish, lead to illness and disease.

A large amount of organic waste in the tank also consumes a significant amount of oxygen. In tanks where there is a lack of flow or surface agitation, the increased microbial activity can also lead to the formation of a biofilm on the water surface, inhibiting gas exchange and further reducing oxygen levels in the tank. All of this will stress the fish and affect their health - and all of this can easily happen in a tank where the water parameters are "perfect".

Organic waste levels are easily increased in tanks that are overstocked or where there is heavy feeding of live food. For this reason, many professional fish keepers who feed their fish heavily carry out regular water changes to keep water quality high.

Shrimps and other detritivores are excellent at breaking down excess food and other larger organic waste particles in the tank.

Having adequate filter media

Ammonia is not the only contaminant in aquarium water. Organic waste gives rise to pathogenic microbes if it is not broken down quickly. Fortunately, filter media harbor microbes that form bio-films that clump together fine organic waste and other contaminants. It is this bio-films that keeps the water crystal clear with low bacteria count in a mature aquarium with adequate filtration. Even very fine filter media alone would have a hard time clarifying the water without the help of microbial bio-films.

The volume of filter media required for ammonia oxidation alone is surprisingly small due to the efficiency of the ammonia oxidising microbes. However, the amount of media required to polish water to high clarity and low bacteria count is significantly more. Thus, ammonia cycling alone cannot be used as an accurate indication that filtration is adequate. While doing bacteria counts of water samples is beyond the scope of hobbyist aquarists, there are many other signs that can show that filtration is inadequate.

Cloudy water that persists over a long period of time, bad odors and persistent thick oily films on the water surface are all tell tale signs that filtration is weak or that the biological system of the aquarium is still immature. Biologically immature systems trigger algae more easily in the context of planted tanks - and diatoms will be more common. Fishes that look constantly stressed or get sick easily despite the tank having "perfect parameters" are another tell tale sign.

Consistently crystal clear water should be the default state of the aquarium when the filtration system is properly set up.

Substrates, especially soil-based ones such as Aquasoil, also provide a large amount of beneficial bacteria and also act as an effective trap for organic debris. Larger gravel, such as pea gravel, suffers from poor surface area and does not work as well.

There are other impact factors for effective filtration, such as having good oxygen levels and adequate flow. Click here to read more in-depth on tank cycling and filtration.

Other fish stress factors

Fish can also face other stress factors even if measured parameters fit their needs perfectly. This includes harassment from other fish, which can be exacerbated by a lack of hiding place/cover, or environmental stresses such as overly strong flow for species that prefer still water. Too small spaces for fish species that need more room to roam can also be stressful. Cumulative stress can result in illness or a reluctance to feed, and lead to deteriorating fish health. Many aspects of the environment can affect fish health besides water parameters.