Anoxic / Toxic lower zone with strange boundary layer?

[Gut Funk]

Good day!

I have had 100% success with my shrimp tank at work, it's been going a year now and I have had many berried shrimp and reared babies!

I decided to start up a tank at my house, a small 3 gallon tank, exactly the same way I started the tank at work that's been so successful, the typical add gravel, water, reef salt, shells, wait one month and put in shrimp.

The problem is now there is a very strange 'layering' going on in my tank. I've taken a few courses in aquatic sciences so I know that bodies of water typically form different layers at different depths, but I didn't' think this would occur in such a small aquarium. There is a very clear 'line' (probably a bacteria layer) in the water about 3 inches above the gravel that separates the water into two layers. The shrimp never venture below the line to the gravel below, and always hang out at the top of the tank, the snails exhibit the exact same behavior. Just out of curiosity, I took some of the water from the lower layer to look at in the microscope, and it really smells quite awful. The upper layer doesn't have a smell to it at all, however. I'm perplexed since I don't see any layering in my work tank and my shrimp happily hang out at the bottom to feed on the algae on the gravel. At home, however, even though the gravel is quite covered in algae, the shrimp have no interest in venturing into the lower layer of the tank.

Anyone else experienced this? Will it go away? I'm afraid to mix the water up just in case it ends up making the whole tank toxified. Thanks for any input!

[Mustafa]

Do you have any pictures or video of this? What you're describing sound more like salinity layering. Higher salinity at the bottom and lower on top. Your tank should not be toxic or anoxic unless you had tons of organic matter in your substrate/water that's now rotting.

[Gut Funk]

Now that you mention it, that makes sense too. Should I just leave it be or gently mix the water?

Here is an image: the layer boundary isnt just on the glass, it runs through the whole tank.

[Varanus]

If I were you I'd gently mix it. These shrimp seem happiest foraging on surfaces, so it'd be good if they have access to the bottom.

[Mustafa]

Hmmm...you *do* seem to have lots of organic matter in your tank. Is that Cyanobacteria growing on your substrate and decoration? Do you have some kind of freshwater shrimp substrate in your tank? Something is making the cyanobacteria grow, and it's usually excess organic material.

[Gut Funk]

Thanks for your replies! I appreciate it.

The only thing I've added so far to the tank was some fine grained black gravel from the pet store (from a factory sealed bag, rinsed), some shell substrate from my work tank, A sterile decoration, and about 4 ounces of water from my established work tank. The tank was started around march 15th, so it's only been up 2 months at this point. I haven't fed the shrimp anything yet.

I think what I will probably do is some water testing of both layers as well as check out what's growing in that layer in the microscope. I want to find out what kind of stuff is living on that layer so I can see what to do, since I haven't checked out if it is cynobacteria yet.

I'm not entirely sure where the extra organics came from but I'm willing to do whatever it takes to fix any problems my tank might have.

Edit: I guess now that I think about it, perhaps the shells and substrate that I took from my work tank had some organic waste in it from the shrimp, so maybe that has something to do with this.

Edit 2: I will take water samples to work tomorrow(currently where the microscope is) for more testing. I have a ppm water tester at home and the upper layer has 1540ppm, whereas the layer below the line is 3400ppm, so at the very least there is a salinity difference here. I will update if I find anything interesting in the microscope.

[Gut Funk]

I'm not really seeing much in the microscope aside from generic algae growth in the substrate. It still stinks terribly when I take from the lower layer of the tank. I suppose what I'll do is just take the entire upper layer of the substrate out and lower the light intensity of the LED, and hope the issue starts to clear itself up. It seems like the aquarium isn't quite balanced correctly and some kind of bacteria or something went crazy and grew faster than anything else in the lower layer. I'm not really sure at this point, but I'll hopefully have this problem cleared up eventually. Today the layer boundary was about half an inch lower than when I had photographed it before, so I'll be keeping track of that as well.

Edit: overnight one of the shrimp didnt make it, so Im going to be transferring all the shrimp to the work tank that is still running well. I dont really understand what happened but I may just restart the whole tank from the ground up without adding algae or anything and just giving it a few months to cycle instead.

[mike.d]

I know this is a few days late, but I'm wondering if you can siphon out the water in the lower layer then fill the top layer with new brackish water. Mix it up well and check the salinity.

Then again, at this point it's probably better/safer to start over.

[Gut Funk]

I know this is a few days late, but I'm wondering if you can siphon out the water in the lower layer then fill the top layer with new brackish water. Mix it up well and check the salinity.

Then again, at this point it's probably better/safer to start over.

It might’ve been something I tried if I didn’t have an alternative, but fortunately the work tank is really solid. The shrimp seem much happier as they are actually swimming about actively and grazing.

I’ll be starting over for sure, I believe this whole problem was 100% related to me trying to give the tank a head start with some algae covered substrate from the work tank, as I can’t figure out how else the system went out of whack. I’ll update once I’ve redone everything and confirmed the tank is ready for try 2.

[Mustafa]

The other possibility is the substrate still. Even though you rinsed it, there may have been heavier than water particulate organics in there (plant pieces, dead bug pieces....who knows). Rinsing does a good job of removing very light and immediately water soluble organics, but unless you soak the substrate for days, it won't really remove everything.

In any case, I agree that starting over is probably the best solution. How is that going?

[Mustafa]

Also, how come your highest salinity measurement is only 3400 ppm? That's almost freshwater.