I'm so confused about dark sand patches

[cthulhuspawn]

I'd really appreciate some experienced advice. I have a 55 gallon reef tank with about 4" of fine sand. The tank has been running with 45g of live rock and a fairly minimal bioload for about a year now. All the water levels are perfect, and my animals seem to be healthy.

Ever since I began I've seen the slowly growing dark patches just underneath the sandbed surface. It's gotten to the point where I can tell that practically the entire underbed is one large dark stratum (scrape away a few centimeters anywhere and the dark sand begins). I do not suffer from calcium sublimation into the sand bed (i.e. "the crusties"), although it does compact itself over time.

I have heard two things about these patches: they are hydrogen sulfide-producing bacteria and should be stirred frequently by either myself or lots of sand sifting organisms (I have a few but not alot)....

....OR they are nitrificating bacteria and should not be disturbed. I do see tiny bubbles that form just above the dark sand stratum, but I suppose that could be either type of gas. Stirring the sand does not seem to produce an odor, but my nose might not be sensitive enough to catch it. When the dark sand is stirred to the top, it slowly lightens until the grey/black color disappears completely after about 15 minutes. However, I could stir the entire tank (clearing the water rapidly with my diatom filter) and the grey sand would reappear within 3 days to a week just as fierce.

I've also heard that color may matter. If so, the patches are definitely a very dark grey, but not fully black.

I'm concerned that maybe I should just toss out the fine sand and go for a larger substrate. It would be a shame to remove any nice organisms I've cultivated so far though (I see some tiny worms, stars, and copepods growing in it). I am willing to go that far if necessary. Have I done something wrong to create this situation that I could do differently with a fresh sand bed?

Thanks in advance for your help; this community is really great and is largely responsible for most of my success! I look forward to learning!

[jenglish]

generally hydrogen sulfide starts at the very bottom. I have never seen it myself because of all the DSB I have seen, I have never actually known anyone to get hydrogen sulfide. Is there any chance of getting a pic?

[Reeftanker3295]

My 10 gallon has like sugar grain sand and it turns black underneath my plants. I dont know what it is but would be nice to know.

[cthulhuspawn]

Here's a photo, thanks for your help!

[luther1200]

That looks pretty typical to me. Most pictures of DSB tanks have a similar look. If it was hydrogen sulfide I think you would know it. The odor is pretty strong I think, plus if you have stirred it I think you have had a casualty if it was hydrogen sulfide. I once read a thread were a guy had a build up of it in his mag scraper and it almost wiped out his whole tank. If that small amount can do that, I think if your whole SB had it it would be all over for any life in there if it was disturbed.

[jenglish]

Is it possible that it is just a ton of detritus? It looks like what I have found in a tiny layer on the very bottom when changing a tank. Hydrogen Sulfide typically is more like little areas that spread out. It is hard to believe there is that much of a anoxic layer. By all accounts if you had that much and stirred you would know it. I would try a thorough cleaning and see if it has an effect.

[cthulhuspawn]

Originally Posted by jenglish

Is it possible that it is just a ton of detritus? It looks like what I have found in a tiny layer on the very bottom when changing a tank. Hydrogen Sulfide typically is more like little areas that spread out. It is hard to believe there is that much of a anoxic layer. By all accounts if you had that much and stirred you would know it. I would try a thorough cleaning and see if it has an effect.

I'd be open to the idea except that if I stir the dark sand it slowly dissolves away and then a few days later the exact same layer reappears. In other words, it "grows" for lack of a better term rather than "collects."

[luther1200]

Do you have a mictoscope? You would be able to see if it was bacteria under a microscope I would think.

[cthulhuspawn]

Originally Posted by luther1200

Do you have a mictoscope? You would be able to see if it was bacteria under a microscope I would think.

No, no microscope in my home yet. I guess I'll leave it alone. I just can't get definitive answers but if noone's dying I guess it's not killing. I just hope it's not a time bomb...

[Elegance Coral]

Originally Posted by cthulhuspawn

I have heard two things about these patches: they are hydrogen sulfide-producing bacteria and should be stirred frequently by either myself or lots of sand sifting organisms (I have a few but not alot)....

Tiny critters in your sand can not keep it clean for you. I know you've probably been told that they will, but it's not going to happen. Simply stirring the sand bed and removing suspended particles with a diatom filter will help to some extent, but it will not cure your problem. As you have noticed already.

....OR they are nitrificating bacteria and should not be disturbed.

This is another myth. There are three major types of anaerobic bacteria. Obligate anaerobes, aerotolerant anaerobes, and facultative anaerobes. Obligate anaerobes can not be exposed to O2. We can think of these like obligate cave dwellers. There are many organisms that have spent such a long time evolving inside the perpetual darkness of a cave that they can no longer survive outside. Like cat fish that have lost pigment in their skin making them appear pink. There is no need for pigment when you live in eternal darkness. They also lose their eye sight or some species even lose their eyes all together. A pink, blind catfish would not survive long in the typical stream or pond. Obligate anaerobes are the same way. They have spent countless years evolving in the depths of swamps or deep nutrient rich substrates where they never come into contact with O2. Just ask yourself, what are the odds of a bacteria from such an environment being collected for the aquarium trade? On top of that, what are the odds of one of these bacteria surviving the trip from the wild, in the south Pacific, to a reef tank in Kansas without contacting oxygen allong the way? Even if by some miracle some of these bacteria did end up in our sand beds, we don't care. We have aerotolerant and facultative anaerobes. These anaerobes can tolerate exposure to oxygen. You're not going to wipe out a population of these bacteria simply by disturbing them to clean a sand bed on occasion.

When the dark sand is stirred to the top, it slowly lightens until the grey/black color disappears completely after about 15 minutes. However, I could stir the entire tank (clearing the water rapidly with my diatom filter) and the grey sand would reappear within 3 days to a week just as fierce.

I believe this is caused by decomposition. When you disturb the sand bed, quite a bit of the detritus and dissolved organic matter (rotten liquid) gets released into the water collumn. This lightens the color of your sand but doesn't eliminate the detritus (solid particles). In a few days time the bacteria dissolve more of the detritus giving the bottom, stagnant layer, of your sand that gray color.

I'm concerned that maybe I should just toss out the fine sand and go for a larger substrate.

I agree.

It would be a shame to remove any nice organisms I've cultivated so far though (I see some tiny worms, stars, and copepods growing in it).

These organisms can survive in your LR and a new cleaner sand bed. Their numbers will be directly proportionate to the amount of food available though. If you replace your sand bed with a clean one, the population will be decreased because the available food supply will be decreased. If you have small critters now, you will still have small critters after the sand is replaced. You just won't have as many. You may even lose some species. For me, providing a healthy environment for my critters like coral, anemones, and fish is more important than culturing tiny critters in a sand bed. There are many different types of tanks though. You will need to decide what's more important to you. An abundance of tiny critters, or healthy coral, fish, and other inverts.
Most of the pets we buy require a very clean environment. Most of the tiny sand bed critters require a filthy environment.

I am willing to go that far if necessary. Have I done something wrong to create this situation that I could do differently with a fresh sand bed?

It's not your fault. You fallowed the advice given. You, like many other hobbyists, just got some really bad advice.

There are steps you can take with a new sand bed to insure that this doesn't happen again. IIWY I'd pull that sand bed. Then replace it with a slightly larger grain size. Very fine sand like yours can be difficult to maintain. Like you have noticed. You can't kick the flow up to high or you'll get a sand storm. Flow is important to keep particulate matter in suspension so that the filtration can remove it. Otherwise, it settles in the sand and rots. I'd buy a simple gravel vac and begin vacuuming the sand during water changes. This doesn't work very well with real fine sand. Simply stirring the sand just releases everything into the water of the aquarium. Much of it will simply settle back out. Stirring doesn't remove near as much detritus as a gravel vac will. A gravel vac removes the detritus without exposing all your expensive pets to as much decomposing organic matter.

Originally Posted by cthulhuspawn

I guess I'll leave it alone. I just can't get definitive answers but if noone's dying I guess it's not killing. I just hope it's not a time bomb...

I tried to give you some definitive answers. If you have any questions, please feel free to ask. That's what we're here for.

I'm sorry, but it is a time bomb. A sand bed like yours doesn't just get better on its own. Over time, the accumulation of rotting organic matter (detritus) will simply get worse. Water quality will degrade, algae will begin to grow, and the health of your pets will suffer. This happens over and over again in this hobby. One of the reasons why BB tanks keep gaining in popularity. The only way to keep a sand bed form becoming a time bomb is to keep it clean. HTH.

[cthulhuspawn]

Thank you Elegant Coral! I'll replace the sand with a slightly-larger grain size at my next financial opportunity! Have a great day everyone!

[cthulhuspawn]

In case anyone is interested, I pulled out all that sand. What a chore! However, Elegant Coral and the others were absolutely correct; the stuff stank to high heaven and was collected detritus rotting away.

I replaced it with 1-2mm grain sand and it already seems to be keeping much cleaner.

I did have to use fine live sand for my tube anemone, but it's planted in a PVC pipe on one side of the tank.

Thanks for all the help everyone!

[jenglish]

Glad to hear it

[Loverotties]

Yeah!

[luther1200]

Good thing you got it now, before it caused a problem.