Air bubble in substrate!

[Youngming]

Hi all, recently i just set up my 1.5FT tank and is cycling for about a week. I noticed that there are lots of air bubble trapped in my substrate and when it's disturbed, the bubble will float up and form into little black pearl on the surface. Will like to check is that normal as this is my first time setting up a 5cm high substrate.
Below are my steps during set up:
1. thin layer of bornewild shrimp soil about 1cm.
2. mosura old sea mud powder.
3. add the rest of soil to reach 5cm height.

And also why the substrate is in 2 tone(2 different colour). Top layer is lighter in colour even it is from the same bag of soil.

[Urban Aquaria]

Thats quite a normal phenomenon, especially in new tank setups, the bubbles are the hydrogen sulphide gas by-products of anaerobic bacteria inside the soil substrate... it tends to form in deeper soil because there is less circulation and oxygen in them.

Usually if you have rooted plants, their roots will help aerate and bring oxygen down into the soil, so there will be alot less of those bubbles being produced.

If you don't have much rooted plants, then either just leave it alone or use a stick to poke the substrate to release the bubbles during regular tank maintenance... do be careful when doing this as the gas can sometimes build up to high concentration if not touched after a long time and the large release of gas could potentially harm sensitive livestock, including emitting a very stinky "rotten eggs" smell into your room.

Your soil substrate having 2 different colors is due to the aquarium light illuminating the top layer, while the bottom layer is blocked from light so it looks darker.

[Youngming]

Thanks for the reply UA. I doesn't have many rooted plants as you can see in the picture. Has been poking the substrate this few days and clean up the surface but there are still plenty of them in it. Don't dare to over do and disturb the substrate as i am still cycling the tank. Will continue to do it during my regular maintenance.

[Dscheng]

Is that a sponge filter with bio media too? So basically you just need a air pump and the sponge filter right?

[Youngming]

Yes. that's the new BIOSPON. Got it from GC.