Bubbles From Substrate

It is possible to get build up of gasses in the substrate. One way to stop pockets forming is to manually 'rake' through the substrate when doing a water change, or get Malaysian trumpet snails. But you have mixed substrate so if you did either of those you'd end up with a homogeneous mixture of everything, and I assume you want layers.
But if the bubbles are only in the sand area, you could try raking through it with your fingers taking care not to disturb the compost/gravel area.

You'll often read that the gas is hydrogen sulphide (the bad egg smell) caused by the breakdown of sulphurous compounds in the sand. These H₂S pockets are often blamed for fish deaths. But if your amano shrimps aren't keeling over it's not hydrogen sulphide. [Did you know that hydrogen sulphide is more poisonous than hydrogen cyanide? It's just that hydrogen sulphide is so smelly that a miniscule amount smells like an awful lot]
Do the bubbles come from somewhere right next to a chunk of decor? It is possible for nitrate eating bacteria to grow in areas where there is no oxygen. These anaerobic bacteria turn nitrate into nitrogen gas, but they can't grow in sufficient numbers to totally remove all the nitrate from the water.

The description for how a tank should smell is "earthy".

If you had hydrogen sulphide pockets, you'd smell them, and it's horrible - the most rotten eggs imaginable. I once used hydrogen sulphide from a cylinder (chemistry research) so I know how horrible it is.

In the area with compost and gravel, it could be something in the compost decomposing. In that scenario it would most likely be carbon dioxide.


Mrs Cookie sounds like Mr Sue 

I do recall that tanks with soil substrates produce small amounts of carbon dioxide which also helps boost plant growth.

I think mineralization takes a few months to happen and then the co2 production will stop. I'm no expert on dirted tanks though!

Thanks Matt & Geoff... 

My set up is just a wee bit shy of two months in now...  Interestingly, this set up has right from day one been super stable in terms of parameters, with just the addition of Tetra SafeStart to help it along...  I see so often on the forums that so many new folks can at times have a real struggle cycling their new set ups - in this sense, I feel that I've been really lucky and am consequently a keen fan of dirted & planted tanks...  Having said that, there are other issues that I have encountered along the way, so I guess it would be fair to say there are swings and roundabouts... 

I don't really mind the gas bubbles as such and it's more a question of curiosity as to what's going on in there than anything else, although I won't mind when they stop or subside either... 

Mostly minor stuff really, depending on outlook...  My first issue was essentially my own fault on the one hand and was almost certainly a textbook newbie mistake:  I took a liking to Black Phantom Tetras, particularly that "eye" they have on their flanks...  So, one day whilst still in the process of stocking the tank, I went to my LFS and took Mrs Cookie along...  They had no BPTs in stock on that visit but Mrs Cookie liked the Red Phantom Tetras and consequently I made what was essentially an impulse purchase of 6 x RPTs without knowing much about them...  These little blighters do nothing but perpetually chase each other and bicker constantly, which I don't find fun to watch...  So, after chatting to the guys up at Wharf, they agreed that I could return them and swap for something else...  Here's where the issue now crops up:  I spent an entire day trying to catch them, employing various techniques, and failed miserably because there are just too many hiding places with all the plants in there, so now I am stuck with the RPTs and their constant quarrelling...  I wouldn't mind, but I was warned about impulse buying fish and still fell into that trap...!!!  Doh...!!!

Next thing was the random arrival of micro-critters...  These include FW limpets, small white worms (round ones - not, thankfully, planaria) and diddy little darty/swimmy insects, real small white things...  I did also see, just once, a small white worm that moved along the glass in that same slug like fashion that planaria do, but it too was round looking and not flat and distinctly shaped like a planaria...  Anyways, the main little white wrigglers soon get eaten if the fish see them, as do the swimmy darty critters...  The limpets get eaten too, if I squish them and they fall through the water column, but not when just cruising on the tank glass...  I should say, that after an initial full on panic at seeing this lot, their numbers are minimal and you rarely see them, even when seeking them out, and I am led to believe that they can just be considered part of the clean up crew really, and a source of free food, especially for the Corys for example...  I have bought some "No Planaria" just in case that one off was a juvenile, even though it didn't look like it was, but won't use it unless I can definitely identify a case of those horrid things in the tank - just good to have on hand and to be ready...  I have, as you can imagine, tightened up my routine maintenance with a view to reducing the detritus and reduced the amount of feeding - which I thought was already pretty small anyways tbh...  So, no major outbreaks or anything, but random micro-critters that may well have arrived in the John Innes #3 possibly...?

This also points to a further potential issue in that because there are a good few plants in the tank, there are many areas where it's just not possible to get the gravel vac in there...

Lastly, the mixed substrate thing with the sand bar (as I call it) at the front...  It was always my plan to get some Corys at some point and that was the logic behind the sand area, and everything was fine until I put the Corys in there, lol, as the little blighters rummage around so much that it's a constant battle having to tweezer the gravel off the sand manually and regularly...  That never happened when it was just Tetras, Amano shrimp & Rabbit snails, but those Corydoras are proper little busybodies on their quest, which is almost certainly while seeking out the blinking micro-critters   

As you see, it's not so bad, maybe not actually bad at all - depending on how these things are perceived, and aside from the pesky Red Phantoms, I wouldn't change a thing...  Just wish that I had gone with Harlequins instead of RPTs, but c'est la vie... 

...and a tip I picked up from a member on here a while back was to use a green net as the one to move the fish into as they're more likely to perceive it as safe (like plants).

I will add one caveat about Phantom Tetras...  the fins on the males when they are posturing to/at each other really is hugely impressive...!!!  Also, no actual damage ever gets done, for all of their chasing and harassing each other that they do...