The Dutch translation of this article should be proofread.
Translated in Dutch by Joost Abrahams

4.3. Free Floating Bacteria

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Author : David Bogert

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Time To Read :
5 minutes
One very overlooked aspect of water chemistry is the number of bacteria found in the water of the aquarium. The bacterial water count is probably the most important part of keeping a fish healthy in an aquarium. All fish are healthier in water which is very clean, with very few bacteria in it.

Note that “bacteria free” is relative on something called a logarithmic scale. A milliliter (or cubic centimeter) of dull, “normal” aquarium water will have roughly 10⁵/ml bacteria (100,000 bacteria per milliliter). A milliliter of cloudy water can easily have a total bacterial count of 7 x 10⁶/ml bacteria (7,000,000). And a milliliter of “crystal clear” water can have as little as 6 x 10³/ml bacteria in it (6,000). So “bad” water has 10³ or 1, 000 times more bacteria that “good” water. This is what “logarithmic” means.

This gives one an idea as to why reducing the bacterial count can be so effective in improving the immune systems of the fish.

picture of an aquarium fish Melanochromis cyaneorhabdos (Maingano)
Melanochromis cyaneorhabdos – Maingano

One often sees where fish are dying one a week or one a day for no apparent reason. If a lot of biofiltration surface area is added or a UV unit is added the problem goes away. What happens here is that the fish’s immune systems can only handle just so much at once. If the organism count in the water is high, then fish must devote significant immune resources to the organisms in the water. This leaves fish open to attack by all sorts of pathogens.

If the bacteria count in the water column is low the fish can put all their immune system resource towards keeping pathogens at bay. So fish in bacteria free water are far more healthy that fish in bacteria laden water.

The Key to Good Fish Health is Clean, Bacteria Free Water

Note that , contrary to popular myth, this does NOT mean water which is changed frequently. Going from ten million bacteria to five million bacteria isn’t going to be that much help to the fish. Large water changes cannot compensate for poor filtration because of the way bacteria are logarithmically present in the water.

Picture of an aquarium fish Tropheops Macrophthalmus Albino
Tropheops Macrophthalmus Albino

Blackwater Fish

This bacterial account is very important for some varieties of “difficult” fish. One well intentioned but ill-informed individual lamented that the neon tetras found today in the pet store have been bred to where they are very fragile and don’t survive in the aquarium. They say, “It’s a breeding problem”. This is incorrect.

Neons tetras are from the Rio Negro which is a blackwater river. This river is very acid and very soft in its make-up, like a pH of 4.5 to 6.0, virtually free of salt and with a dGH of 1. Everyone incorrectly thinks the pH is what is important. It is the dGH and salt level and their effect on bacteria which are important, not the pH.

What people don’t realize is how incredibly clean the Rio Negro is. The water has very few bacteria in it. The water has few bacteria in it because bacteria require a dGH of at least 3 and some salt to survive. Indeed, 90% of the world’s freshwater fish species would die in the Rio Negro because of the low dGH and low salt. Their gills simply cannot prevent the salts inside the fish from being sucked into the surrounding waters.

Neons are fish which should never be in anything but very clean, crystal-clear, bacteria-free water in an aquarium which has been established for at least four months. Yet many unfortunate hobbyists keeps putting them in the bacteria laden water. Then they say that breeding is at fault when they die. The same holds true for rummy nosed tetras, hatchet fish, uaru, rams, apistos and discus.

picture of an aquarium fish Sciaenochromis fryeri
Sciaenochromis fryeri

Getting Bacteria Free Water

There are several ways to get relatively bacteria free water:

  • Use over-filtration, at a rate of at least 100 ft²/ft³ (330 m²/m³) of biofilter surface area per pound (454 gram) of fish, established for at least four months without cleaning (this is required even with the two elements listed below).
  • Add a UV light at a rate of one watt for every ten gallons (40 liter) of water, with reservations (see below).
  • Add some fine polishing filtration which can remove the bacteria by mechanical filtration, with reservations (see below).

Water changes do not significantly reduce the bacteria counts. Surprisingly UV also often does not reduce the bacterial count in the water. The bacteria can reproduce so fast that they can typically out compete a UV light. It will all depend on how often the water goes through the UV unit. A 2X turnover might not reduce the bacterial count while a 8X turnover might well significantly reduce the bacterial count.

Note that UV treatment and polishing filtration cannot eliminate dissolved organic compounds (DOCs). Over-filtration can eliminate DOCs. Since DOCs are what feed bacteria in the water column it is important to eliminate them to produce really crystal clear water that is free of bacteria. So UV and polishing filtration need to be accompanied by very good standard media filtration to produce crystal clear, bacteria free water.

Tropheus sp. Black Caramba Red Bishop
Tropheus sp. – Black Caramba Red Bishop

We discuss how to obtain low bacterial count water in depth in “Crystal-Clear Water”. The same things which clear the water also reduce the bacterial count.

6.4. Crystal Clear Water

Also UV sterilization can sometimes be very useful in the fight against bacteria in the water column. This is explored in more depth on this web page:

14.1.1. UV in More Depth