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There is a lot of confusion in the hobby today about what bacteria do in the aquarium. Which ones are responsible for what, how do they work and on and on. Hopefully this will help straighten out some of that confusion.
First, lets get some definitions out of the way.

Autotrophic. Autotrophic bacteria are those that synthesize organic substances from inorganic molecules by using light or chemical energy.

Heterotrophic. Heterotrophic bacteria are those that directly process organic substances.

Aerobic. Aerobic indicates that the bacteria requires oxygen to survive. Anaerobic is the opposite. The bacteria do not require oxygen.

Now, what does this have to do with the aquarium hobby? Everything. The environments we set up in our tanks are dependent on the functions of these bacteria. Without them, we would simply have glass boxes full of dead fish and ammonia.
If you have been in the hobby for any length of time, you have at least heard of nitrifying bacteria. These are autotrophic bacteria that process ammonia to nitrite to nitrate. The ammonia and nitrite are their food, and the chemical process they use to break them down is how they get their energy. These bacteria are also aerobic, and must have a good source of oxygen to function to their fullest extent. This is why wet dry filters are so good at biological filtration. Also, these bacteria are not free floating. They attach to any available surface that will provide them with food and oxygen. The more of both that the surface provides, the more bacteria will colonize there. Their reproduction rate is about once every 24 hours.
For years it was believed that the species of bacteria responsible for nitrification were Nitrobacter and Nitrosomonas. It has recently been shown that this is wrong and that Nitrospira is actually responsible. As stated, these bacteria require oxygen to survive. They do not encyst, which is a process some bacteria go through when conditions are unfavorable. The bacteria form sort of a case around themselves and go dormant until the conditions around them are again favorable. This is important to understand as it shows that the bacteria in a bottle products out there are not all they are cracked up to be.
There is a very important second set of bacteria a hobbyist needs to know about. These are the heterotrophic bacteria. These are free swimming bacteria that consume any dissolved nutrients in the water column, and also help to break down such things as dead fish, excess food and plant material that is not under gravel or in anaerobic conditions. Heterotrophs can be both aerobic and anaerobic, but the ones we are concerned about here are aerobic.
The heterotrophs are the bacteria responsible for the occurrence of what is termed a bacteria bloom. This is where the water in the tank becomes cloudy as if milk was poured into the tank. Many people mistakenly attribute this to the development of the nitrifying bacteria. This is understandable, as they do tend to go hand in hand, but it is erroneous.
As defined, the heterotrophs feed on organic substances. They also reproduce at a rate of about once every 20 minutes. As with any organism, they will populate according to the amount of food available. In a new tank, there is a sudden increase in nutrients available. Especially if the hobbyist is new to the game and overfeeds. With this rapid increase in nutrients, the heterotrophs reproduce accordingly, and they become so numerous that they cloud the water, leading to the bacteria bloom. As you can see, this is independent of the establishment of the nitrifying bacteria. This will also occur in well established tanks, especially after a water change. When you vacuum the gravel, you free up nutrients that were not available to the heterotrophs previously. Again they reproduce accordingly and you have cloudy water. And again, this is NOT connected to the “cycle� or the nitrifying bacteria.
I have stressed that the heterotrophs are not a part of the “cycle�, and this is true, at least so far as most people consider it. However, the heterotrophs do indeed come into play, as does anything in the aquarium that produces ammonia. The heterotrophs produce ammonia, so they do feed the nitrifying bacteria to an extent, but that is about as far as it goes.
The heterotrophs have another quality that comes into play in our hobby. If need be, they can actually adapt to process ammonia as a food source. Now, this does not normally occur in anything but extreme conditions, but it does explain how some people report some success with the bacteria in a bottle products. The heterotrophs present may indeed process ammonia initially for a short period, leading the hobbyist to believe nitrifying bacteria have been introduced in large numbers. This is not going to happen every time one of these products is used, but it will occasionally.
Hopefully this has given you a further insight to how our aquariums work.

Very nice Double D!

Thanks chc.
I wrote it for the specific purpose of simply being able to post a link for all the cloudy water questions that pop up. Hopefully it will get moved to the saved topics section here soon.

good work DD.

100% correct.. like usual.

Great read, thanks man.

question:

will bacterial bloom cause harm(s) to aquarium inhabitants?

Very nice. I think I will leave this here for a bit longer, then move it to saved topics.

Thanks for the compliments guys. Just glad it didnt confuse people further.
phatboy, in and of itself the bloom is harmless to your fish. However, if the bloom is being caused by something like a hidden dead fish, accidental overfeeding or the like, the ammonia levels will likely be high enough to cause problems.
Granted, the bacteria are producing a lot of that ammonia, but only because there is something in the tank elevating the nutrient levels. The way I look at it, the bloom is the symptom, not the cause.

agreed.

people often try and treet the symptom and dont bother looking into the cause.

good topic...here's a link to get further in depth. scroll down to the area marked "introduction" bacterial roles.

Great article!

great write-up!
I've forowarded the link to a few of my aquarium friends.

Well this was posted 2 years ago. But I think it helped me find out the reason why my ammonia level was so high. Thanks!

This post has been edited by bluehairman: Aug 2 2007, 11:13 PM

darn good info thats for sure

Well stated! I do understand the reasons of change in water conditions. Thank you Donald!