Dark discoloration on sand bed

[Andos99]

I've had pool filter sand in my 50 gal for about 5 months now and I love it. Starting about 6 weeks ago or so, the entire surface of the sand turns a far darker shade then the sand itself. This process takes about 7-10 days after a gravel vac. I'm assuming it's a layer of bacteria the serves as ground cover, but I'm not sure. When I do my water changes, I sift the sand so it goes back to it's original state and then the layer grows back. So far I haven't noticed any detrimental effects on my fish or parameters. Any ideas or comments? TIA

 

[Fruitbat]

Andos99...

Is the darker coloration a brownish or brown/orange? If so then it is likely diatoms...sometimes called brown algae. This is a very common situation especially in low-light or newly set up aquariums. Unfortunately there is very little you can do to prevent the problem from recurring but on the bright side the little diatoms aren't harmful.

-Joe

 

[krap101]

maybe anaerobic bacteria?

 

[Fruitbat]

If they were anaerobic bacteria then they certainly wouldn't be on the surface of the sand. By definition, anaerobic bacteria thrive only in places with limited or no oxygen which is why the aquatic species are generally found beneath the substrate surface in very fine-grained, thickly-packed sediment. The amount of oxygen in the water at the sand's surface would be certain death for practically any anaerobes (facultative anaerobes excepted).

-Joe

 

[Andos99]

It's a brownish color. The sand itself is light tan and the surface turns a darker brown color. I'm only putting out 30 watts of light so according to what you said fruitbat, it's probably diatoms. The tank is almost a year old, so I don't think it would be considered a "new" setup. Is there anything that will eat it or keep it under control? Is it also ok, to sift the sand during water changes to mix it up?

 

[Fishyfanatic]

You actually should be mixing the sand at water changes to prevent gas buildup. So I'd say keep mixing it.

 

[Fruitbat]

30 watts of light on a 50 gallon tank would definitely put it in the low light category as far as plants are concerned. Farlowella-type cats (sometimes called 'Twig Catfish') will graze on diatoms on aquarium glass and decorations but I've never seen them take it off the surface of the substrate. Some folks say that Otocinclus species will also feed on diatoms. Other than that....I don't know of a fish that will eat the stuff. I've got diatoms in all of my tanks and it is something I've just learned to live with over the years. Cleaning them off the glass is just one of those never-ending parts of aquarium maintenance, I'm afraid.

-Joe

 

[Andos99]

Gas build up? What exactly does that mean and can it be bad?

 

[Fishyfanatic]

The buildup of gas in the sand, if disturbed, can kill your fish. That's why you have to mix it around. Then you don't have the build ups. If done at every water change or even every other water change, it shouldn't be a problem. Some people use MTS to mix it up for them. We have eels in one tank.

 

[Andos99]

How can a gas build up kill your fish? What kind of gas is it?

 

[mattrox]

Sand can develop anerobic pockets. When you pwc you should vacuum the sand to ensure this doesn't occur. The gas that builds up is hydrogen sulfide (H2S) and is noxic. Not to mention smelly. It is cause because anerobic bacteria use sulfur as their 'energy source' rather than oxygen.