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Do anaerobic bacteria in freshwater do the same as

SirMinion

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It's common practice in marine setups to purposly create a 'dead' area in the sand to allow anaerobic bateria to grow. These bacteria take nitrAtes from the water which they consume and give off nitrogen gas as a waste product thus helping to keep nitrAtes in the tank as low as possible.

In a sand-based freshwater tank, it's easy to develop pockets of anaerobic bactria and gas bubbles if the sand is not frequently stirred and we're given to understand that these pockets are a bad thing.

My question is, do the freshwater anaerobes not do the same as the saltwater and convert nitrAtes to nitrogen gas? Are they really such a bad thing in a freshwater tank?

Anaerobic bacteria gas bubbles in a freshwater sand based tank:
 

Paul_MTS

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since i started and understood marine more i wondered this so should be interesting to see the replies!
 

modernhamlet

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Very interesting question. I don't know the chemistry behind deep sand beds in saltwater tanks, but my understanding is that anaerobic areas produce hydrogen sulfide and other toxic substances, at least in freshwater tank, which is why they are considered undesirable.

Anyone know why this isn't a concern for saltwater tanks?

One idea for freshwater would be to have a deep sand bed within a sump such that any escaped gasses that would normally be dangerous to the aquarium could be released more safely. Any comments on this concept?
 

Synirr

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My understanding was that anaerobic bacteria are only dangerous in freshwater in the event that a large toxic bubble is allowed to build up under the sand and is spontaneously released all at once. I mean, I only bother to stir my sand about once every two months, and sometimes small bubbles are released during the process but no fish have ever come to harm because of it :dunno:
 

CFC

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Anerobic bacteria do exactly the same thing in freshwater, many of the suposed nitrate removing medias rely on breeding a colony of anerobic bacteria to do the job, basicly the media knits tightly together and when placed in the lowest flow area of the filter (the last stage) the low oxygen levels mean that anerobic colonies can form, these then break down nitrate by stealing the oxygen from it converting it to harmless nitrogen gas.
 

Bignose

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Many biological processes generate electrons, and these have to be used somewhere (the organism does not allow charge to build up). The transfer of charge can also be used a source of energy not unlike how we (humans) light lightbulbs for example. Most typically, oxygen can be used as an electron acceptor, since it is normally available, and tends to be the most favorable acceptor (measured by redox potential). The oxygen redox is +816 mV, (+ means it is favorable). Nitrate redox is +421 mV.

This nitrate redox is what the live sand and live rock are performing:

NO3(-) + 2 H(+) + 2 e(-) --> NO2(-) + H2O

This is only the first step, nitrate to nitrite. The overall reaction is:

NO3(-) --> NO2(-) --> NO --> N2O --> N2

Nitrates to nitrogen gas.

When oxygen and nitrates are gone, many bacteria begin to substitute Iron (Fe) or Manganese (Mn) to accept the electrons generated. While there is typically much more Iron than Manganese, Mn is th better electron acceptor:

MnO2 + 4 H(+) + 2 e(-) --> Mn(2+) + 2 H2O

When MnO2 is exhausted, ferric iron can be used to accept electrons:

Fe(OH)3 + 3 H(+) + e(-) --> Fe(2+) + 3 H2O.

Both of these have lessor redox, though. Mn: +396 mV, Fe: -182 mV.

Sulfate can also be used as an electron acceptor:

SO4(2-) + 10 H(+) + 8 e(-) --> H2S + 4 H2O

which has a redox potential of -215 mV But, when nothing else is available to accept the electrons, the organism has to go somewhere with them.

So, if the substrate is anaerobic and has been delpleted of its manganese and iron, it may turn to sulfer (which is in all the foods and is typically very abundant). And note that a by-product of the reaction is hydrogen sulfide (H2S) gas that is very toxic to all living things. The really good news is that H2S is very unstable near oxygen. As soon as H2S runs into some oxygenated water it becomes sulfuric acid, which while dangerous, it nowhere near as bad as hydrogen sulfide gas:

H2S + 2 O2 --> HSO4(-) + H(+)

As a point, H2S was found to be negligible in a study of a swamp with oxygenated water, even when the sediment levels in the swamp were very high. I think that the classic danger that is warned about is H2S gas, but my understanding is that H2S is neutralized very very quickly. So you would have to have quite a lot of stagnant H2S for it to come out of the subtrate and kill something.
 

modernhamlet

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Bignose said:
Many biological processes generate electrons...

you would have to have quite a lot of stagnant H2S for it to come out of the subtrate and kill something.
Solid explanation. :thumbs:

And thanks for the sudden 12th grade AP Chemistry flashback. I spent 4 years of college drinking away all memory of redox potential. Oh well... where's my beer?

:beer:
 

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In marine tanks, it's commonly accepted that a healthy population of fauna is needed for a deep sand bed to be effective at processing waste. These bristle worms/ flat worms/ small crustaceans etc. move the sand around slowly and digest some of the excess detritus or algae, keeping the bed from becoming a nutrient sink.

I'm not sure if it is possible for us to keep similar organisms in a freshwater tank (I have the feeling you may end up with a load of flying insects in your tank room in a few months time if you try), so this might limit deep sand beds use within freshwater tanks.
 

andywg

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As mentioned above, I recall reading that there needs to be some flow around the DSB in marine and as Ed has stated, there are critters to slowly churn the DSB around in marine. Hence the gas bubbles are a less common feature. Also, I recall reading that DSBs are not normally much over 5 or 6 inches as most critters don't burrow much further than that and as such the extra depth gives little benefit.

I think if you set up a sump and could find some accetable worms and others to burrow you could have a good setup. I'm sure I've read of others on the web trying this in FW but don't remember seeing results.
 

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As has already meen mentioned, the key to the success of a DSB is down to the depth of the sand and the life within it. The depth is important so that the anearobic bacteria find place to thrive, (this requires an oxygen poor environment), freshwater is richer in oxygen than salt water so this is not a great help to start with.
Marine tanks are blessed with lots of different micro life and crustaceans that help with keeping the sandbed moving. Bristleworms, pods and other forms of micro life burrow thrugh the sand keeping it moving and slowly bringing these unwanted gases to the surface. If the micro life is destroyed in a DSB then it soon turns toxic and can kill everything in the entire tank.

Im not sure what micro life can be sustained within a home freshwater aquarium to achieve this but if such creatures could be kept then i see no reason why a freshwater DSB wouldnt work.
 
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SirMinion

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Navarre said:
Im not sure what micro life can be sustained within a home freshwater aquarium to achieve this but if such creatures could be kept then i see no reason why a freshwater DSB wouldnt work.
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This is all very interesting!

the only burrowing critter I can think of that's easy to maintain in a freshwater tank is the good old Malaysian Trumpet Snail. We have these in all our tanks (whether we want them or not!) and they do a sterling job in keeping the sand turned over.

We have a deep sand bed in Martha's tank because she likes to sleep under the sand.
It takes a lot of sand to cover a thirteen inch puffer!
Even though the sand is eight inches deep in places I've never had a problem with anaerobic buildup because I stir it once a week and redestribute it for her.

However it's always been a concern that there are some areas that are hard to get to such as under plant roots and bogwood chunks but now it seems that it's maybe not such a big worry after all.

Thanks everyone. :D
 

steelhealr

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Although I obviously have never tried a DSB in a FW tank, and, I could not find much on it in a quick internet search, here are some thoughts on it from my perspective.

The main reason for considering a DSB in a marine system is for denitrification, ie. conversion of nitrates via anaerobic bacteria into nitrogen gas. The construction of a DSB requires very fine sand and usually a layer greater than 3". 4" is the standard recommendation. Jaubert, I believe, gets the credit for this. Other considerations include a plenum which is an artificially constructed framework that creates a water layer within the substrate to provide the area for denitrification.

As Navarre stated above, invertebrates or other animals help to stir up this bed and assist in some aeration and movement of water flow. The fear of hydrogen sulfide, I think, is really unfounded and if it occurs is related to overfeeding, poor circulation and use of the wrong substrate (coarse sand or crushed coral) that will collect detritus.

I think the real answer lies in the differences in physiology of marine fishes vs. freshwater. FW fishes can excrete their wastes in large volumes of water while marine fishes must concentrate theirs to keep 'FW within their systems'. Hence, FW fish can tolerate higher levels of nitrates than SW. A freshwater tank is usually in a good state if nitrate levels are at 40ppm. At the same level in SW, stress occurs, corals close up, algae blooms occur and loss of marine life can follow. So, on an operational/maintenance level, the need for a DSB in marine system perhaps is higher and obviously more functional. FW tanks can be more easily handled with water changes.

Would a DSB function specifically in FW? I can't answer with total confidence. I think probably not well. I will say that tank footprint/size is an important component that I CAN answer since I have researched the use of a DSB on nano systems. Generally, they are not acceptable/functional in nano marine systems as you need a large surface area to function. Hope this adds/helps. SH
 

andywg

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Good to get the extra help from the salty side. Thanks peeps.

I have read a discussion before and it is generally not believed to be wise to trust malaysian trumpets to burrow much below 2" so just relying on them might not be the best idea.

I guess the real answer to FW DSB would be to be able to get an area poor enough in oxygen to get anaerobic bacteria yet with enough smaller burrowing critters to keep it turned over.

-Edit---

I have PM'ed someone on another forum who may be able to contribute to this debate as he has done a lot of good research into alternative FW filtration. Hopefully will hear from him soon.
 
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