*****TRY NOT TO USE ABREVIATIONS FOR NAMES OF FISH, MEDICATIONS, PROCESSES AND OR PIECES OF EQUIPMENT UNLESS THEY ARE INDUSTRY STANDARDS IN WHICH THERE WILL BE LITTLE ERROR FOR MISINTERPRETATION****
In order to determine what may be happening inside your aquarium and/or what may be affecting your fish, please provide a short history of your tank along with water quality test results that you have recently performed. Important water parameters are, but not limited to: ammonia, nitrite, nitrate pH, temperature, general hardness (gH), and alkalinity. (added chlorine to this list also (31 Oct 2008).
If fishes are of concern, please provide information as to species type (common names are ok), approx. (if known), and other fishes in the same tank.
When one or more fish appear to exhibit signs of ill health or unexplained injuries, you should first rule out the possibility of a noninfectious disease or problem. More often than not, health problems are relieved by examining and then properly correcting things that you can control in and around your aquarium.
The first protocol steps to diagnosing fish health problems is:
a. Can the problem be related to aquarium water?
Temperature, nitrogen compounds, pH, toxins introduced into the water such as: (proper dechlorination, unintentional spills or items added to the water. room sprays or cleaning the outside glass with a Windex-type product), source of your water (well, municipal, bottled, etc.), alkalinity of the water.
b. Could the problem be related to aquarium set up and management?
Is the tank a suitable size for the type and amount of fish housed? Filter and aeration? What is the amount and how frequently is the water partially changed?
c. Could the problem be related to choice of fish?
Compatible species? Stocking density for selected species? Where the fish quarantined, and were they previously treated for anything while in quarantine?
d. Could the problem be related to nutrition?
Are you feeding a nutritionally balanced diet? Is the food fresh? Is the feeding frequency appropriate? Are you feeding any live foods, and what was their source?
With those possible issues mentioned above dealt with, Fish exhibiting unusual swimming patterns such as hanging out at the surface, could indicate low oxygen, gill parasites, or gill damage. Fish showing a sluggish pattern and hanging out at the bottom could also indicate parasites, or cytophaga bacteria, or poor water conditions.
( 31 Oct 2008 addition) Other things to consider.................................
"Disease is a deviation from normal health and may result from infectious agents, nutritional deficiencies, toxicants, environmental factors, or may be genetically based (Plumb 2002). There is a difference between "infection" and "disease". Infection is the presence of a pathogen in a host that may or may not be diseased. Infected fish that are not diseased can serve as a natural reservoir for pathogens (Coutant 1998). This is often a normal state. Disease is the condition in which a pathogen is present in sufficient numbers and under the right environmental conditions to affect the animal's well being (Plumb 2002). Many fish disease organisms are normally present in the aquatic environment, coexisting with the host species and natural population without causing regular or significant outbreaks, and/or wide spread mortality (Plumb 2002). However, if environmental conditions become unfavorable for the host and some stressor(s) compromises individual immune systems or natural resistance, disease outbreaks may result."
"An important relationship exists between environmental quality and fish disease. Changes in environmental conditions affect the incidence and severity of infectious disease (Plumb 2002). Fish are constantly adapting to changing environmental conditions that may result in stress to an individual fish (Plumb 2002). Stress is generally defined as a reaction of an animal to physical, physiological, or chemical insult (Barton 1997). Stress can manifest itself in many ways including reduced weight gain, reduced immune response, higher disease incidence, and increased mortalities (Plumb 2002). Some stressors for fish include organic pollution (nitrates and nitrites), low oxygen levels, high concentrations of carbon dioxide, water temperature (based on specific species requirements), fish density, heavy metals, pesticides, high turbidity, and pH extremes (Plumb 2002). Dr. S.F. Snieszko (1973) theorized that a host/pathogen/environment relationship exists, such that if a host and pathogen are both present in combination with an unfavorable environmental condition, disease may result. For example, fisheries management actions such as artificial propagation can be associated with density dependant stress during rearing, which when combined with natural occurrence of pathogens in raceway water can lead to disease outbreaks in the culture environment. However, it is important to note that the presence of pathogen and host infection does not necessarily indicate that a disease outbreak has or will occur."
"Fish health management protocols should consider potential interactions among a broad set of parameters such as maintenance practices, environmental quality, nutritional factors, physiological status of the fish, and specific pathogen attributes (Plumb 2002)".
*Added 6 Sep 2011 regarding feeding*
"The requirements for several B-vitamins increase when fish are treated with antibiotics, because microbes that can synthesize vitamins in the gut are killed by antibiotic treatments. The requirement for vitamin A may increase when fish are exposed to stress". (R. Winfree 1981)
*Added on 23 Feb 2016*
Mysterious fish deaths could be attributed to large amounts of total dissolved solids (TDS) in your aquarium water.