Simple Guide to Aquarium Bacteria (for Newbs like me!)

[Marx]

Simple Guide to Aquarium Bacteria

by Josh Shilling
Tulsa Cichlids Website
This article appears here with the permission of Josh Shilling.

This article is intended to be an introduction to bacteria in the aquarium for beginning aquarists. This is by no means a complete catalogue of all possible bacteria species that could be found in your aquarium. Many people think that the only bacteria in their aquarium are the "nitrifying" bacteria that we all hear so much about. Others think of dangerous pathogens that will harm their fish when they think of bacteria, which is not always the case, either. I am not including a discuss of blue-green algae in this article; even though they are actually bacteria and not algae, they do behave more like a type of algae and as such do not belong in this discussion.

There are basically four types of bacteria found in most aquariums. The first type are best termed scavengers or heterotrophic bacteria. These are bacteria that feed on various items in the aquarium. They feed on fish waste, uneaten fish food, rotting plants, dead algae and diatoms, dead bacteria, and other organic matter that is available as a food source to them. In most aquariums, at least in number of types and species, this constitutues the largest category of bacteria. These bacteria are a mixed blessing- they break down organic substances into metabolic byproducts that can be consumed by other bacteria. They are a sort of "cleaning crew", although most of what they eat is not visible to you or I. It is important to remember these bacteria. These bacteria are the primary cause of cloudy (grey) water. When you overfeed your fish, and they end up gasping at the surface of the tank, it is not really the fish food that is causing the problem. It is the huge increase in population of these bacteria that consumes most of the oxygen, and produces a large quantity of ammonia. If nothing were there to consume the fish food and use up oxygen to turn it into ammonia and carbon dioxide, overfeeding would not be as much of a problem. So, it is useful, especially when devising what type of filtration and stocking rates to use in an aquarium, to consider this additional biomass. These bacteria are like having a few more fish in your tank than you actually do. In proper numbers, they perform a valuable service by helping to clean our aqauriums; when out of control, their last meal will likely be your dead, rotting fish. It is important to provide a margin for error to plan for the worst case; for example, if you went on vacation, and one of your fish died, would the added load of the fish being consumed by the bacteria be too much for your filtration to handle? Many of us tend to stock our tanks to the maximum limit; I am as guilty of this as anyone. But bear in mind, that that one extra fish you are adding, may be the difference, if disaster were to strike, between losing 1 fish and losing them all. Also, these bacteria compete for living space with the important nitrifying bacteria, which means if you overfeed your fish or do not remove detritus from your tank often enough, your biofilter's efficiency will be reduced.

The second type of bacteria found in aquariums are also heterotrophic, but they are more commonly termed nitrifying bacteria. They comprise several different species, depending on your water temperature and parameters. There are basically two subtypes, one subtype uses ammonia and oxygen as a food source and produces nitrite as a biproduct of its metabolism. The second type uses nitrite and oxygen and produces nitrate. Ammonia and nitrite are toxic to freshwater fish in minute ammounts, depending on the pH of the water. The higher the pH, the more toxic the compounds become. At low concentrations, ammonia can damage the gills, skin, and fins of a fish, eventually causing death. Nitrites are not as toxic, but will still lead to death in elevated concentrations. Nitrate, on the other hand, is largely harmless up to relatively high concentrations. So the goal of a biofilter is to quickly and efficiently turn any ammonia excreted by the fish or scavenging bacteria into nitrite and then nitrate as soon as possible. The nitrate is then allowed to build up, and is dilluted by water changes in most cases. De-nitrate equipment and water treatments are available, but are more commonly used in marine aquaria because it is cheaper and easier to discard and replace the fresh water with tap water. Also, even if de-nitrate equipment is in place, there are other metabolic byproducts that build up that would have to be dilluted eventually anyway, such that water changes are inevitable at some point unless a great deal of effort is spent maintaining the water quality of the aquarium (for example, the top off water to replace evaporation would have to be distilled water to prevent salt buildup).

The third type of bacteria are termed as pathogenic. Some of them are the same bacteria in the first category, that will feed on a fish's dead skin when an injury occurs, for example. An analogy to this would be E. coli in humans. E. coli, which is infamous for food poisoning outbreaks, occurs naturally in the bodies of cows and humans. In its proper place, the bacteria does not harm the host and may even be beneficial. However, if you get E. coli in your food, or a pathogenic strain of E. coli, you could be in trouble. This is much like the oppurtunistic pathogens; they do not bother the fish until the fish is wounded or weakened by some other stress. Others are truly pathogens, and can be present in the aquarium at any time. These are constantly fighting a battle with the immune system of the fish. As long as the fish are healthy, the immune system is able to fend off these pathogens. When one fish becomes weak and succumbs to a pathogen, this sometimes gives the pathogen a host to allow it to multiply to large enough numbers that they can overwhelm the immune systems of the other healthy fish. Keep in mind that in an aquarium, the number of fish per unit volume of water far exceeds any found in nature in most all cases. The stocking rates in nature would be like a single neon tetra in a 55 gallon tank. So in nature, when a fish sucumbs to a pathogen, the bacteria are dilluted by this large volume of water and cannot overwhelm the immune system of the sick fish (the same is true for many parasites, such as ick or velvet- it is not uncommon to collect wild fish with a single spot of ich on them). The best way to combat these pathogens is to prevent stress. The second way to control them is to limit exposure by quaranteening new fish; the pathogens that are dependant on hosts for reproduction will eventually dissappear from a tank full of healthy fish. Keep in mind the methods of transport of pathogens are not just fish; nets, filter materials, wet arms, etc may all transport these bacteria. Drying them out may not be enough to kill them in all cases, so use caution. Most good fish stores can give you really good tips on how to prevent cross contamination; because they have so many tanks, and most fish are stressed by shipping, they live on the front lines of this battle.

The fourth category of bacteria are lumped together as anaerobic bacteria. They may include some of the bacteria above; some of them have the ability to switch from operating in a mode that needs oxygen to a mode that does not. The aerobic mode of operation is more efficient, and is usually preferred. The anaerobic bacteria live in areas devoid of oxygen, such as deep in the substrate of the aquarium or in areas where decorations cover the substrate. Some of the anaerobic bacteria are beneficial; some of them convert nitrate into nitrogen gas. This is why most denitrators have such slow flow rates and long coiled tubes- the idea is that bacteria will colonize the first part of the tube or denitrator and consume all of the oxygen, so everything from that point on will be anaerobic. In large quantities, anaerobic bacteria are bad because some types produce hydrogen sulfide as a metabolic byproduct. Hydrogen sulfide smells like rotten eggs; in marine aquaria, just stirring an anaerobic pocket can kill the fish. In freshwater it is largely less toxic, but is a symptom of poor husbandry techniques. These anaerobic areas also indicate lost bio filter real estate. Some of the other byproducts of anaerobic respiration are toxic if allowed to build up, but hydrogen sulfide is the most noticeable. The main way to fight anaerobic areas are to avoid placing rocks and decorations in a way that water cannot flow over the surface of the substrate. Decreasing substrate depth or increasing particle size will allow more oxygen to go deeper in the substrate. Fine sand has a tendancy to get packed and turn anaerobic; livestock ranging from Malaysian livebearing snails to horseface loaches to eartheater cichlids to (small) softshell turtles may be needed to prevent anaerobic pockets from forming.

[Marc]

Hi Marx, since we've received the permission of Josh Shilling to show his article I've restored it to your post.

[Marx]

okay.. again sorry bout that.. i'll know next time!!