Black layer between gravel and compost. Should I be concerned?


A newbie here with no prior knowledge at fish/shrimp keeping and planted aquarium.
Spent a few weeks researching the Walstad method to set up a no-filter aquarium ... no-filter part is still not happening just yet.

My setup:
36L aquarium. Water temperature is constantly sitting at 24 degrees Celcius.
The substrate is made of about 2 cm of sifted organic compost mixed with some chicken manure topped with 2-3mm gravel. The gravel is about 4 cm thick.
Filled up the tank and poked around the substrate to remove any air pockets before performing a full water change.
The tank was left sitting for 2 weeks with the filter running before introducing any plants.
I have planted some Wisteria, Bacopa Caroliniana, Pink baby tears, Dwarf baby tears, Rotala Wallichii, and Rotala Rotundifolia and about 95% survived and is currently thriving.
Added 6 juvenile red cherry shrimps about 3 weeks after the plants were added... it just so happened that there were 3 stowaways baby shrimps, and all 3 have survived till now.
The initial brown algae bloom has been taken care of by the shrimps so I was under the impression that everything is hunky-dory.
LED lights (blue and white) are kept on for about 8-10 hours every day with 2 air stones; one on each side of the aquarium.

I noticed that there is a spot on my gravel around the leftover stump of a dead Rotala Rotundifolia turning black, which I assume is due to the stump rotting.
Removed the stump and washed the gravel before putting them back.
Now I noticed that there is a layer of black between my compost and gravel layer.
There are occasional air bubbles coming up from the gravel every now and then too.
I have tried to poke the substrate to remove any build-up of air bubbles once every few days but I am not sure if that is the long-term solution.

Now that I have new plants (Hornwort, Subwassertang, and Crystalwort) and shrimps (5x chameleon/ninja shrimps and 4x orange-eyed blue tiger shrimps) coming in this week, I am just curious and wanted to know if this black layer is something I should be worried about before putting in the newcomers.
I have attached a full picture of the tank and with a second picture zooming in to the substrate. Not sure why the tank picture is a bit blurry and making it look like there is a misty layer on top of the gravel.
Appreciate any thoughts/advice/recommendations. Thanks!


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I'm not sure what your black layer is but re: the gravel/air pockets, you could look into something like Malaysian Trumpet Snails (MTS) as a possible solution. They corkscrew/dig down into the gravel during the day and come up at night. They also eat leftovers/scavenge like your shrimp would.
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I believe everything continues to be hunky dory.
The black divide between the gravel & compost is likely a mixture of bacteria & algae, doing their job breaking down the organics.
Gas pockets in the gravel will likely occur on a regular basis until the compost completely decomposes. Btw nice looking tank.
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Thanks, folks.
I actually just killed a Malaysian Trumpet Snail that snuck in when I bought the plants. Oops!

A201 It did cross my mind that it might be black algae. It looked like it when I stir the gravel around a little to try and identify what it is. Looks like I can rest a little easier :)
Thank you for the compliment. It was my first set up with plans on reshaping it and adding mopani wood, spiderwood, or crafted bonsai tree topped with christmas moss among a small mountains and cave systems created by stacking and arranging dragon stones.
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If you had one mts, don't worry, you have more.
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