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Hello everyone

I have an 18 Gal. Aquarium, set up 2 months ago. With a substrate mix of peat, humus, coconut shells and perlite, about 1” thick, and a top layer of small volcanic gravel, about 1/2” thick.
I have some plants, which are doing mostly fine and about 15 small fish, some shrimp and trumpet snails.
Everything seemed fine, except for some tannin brownish water, I couldn’t get rid of by charcoal filtering.

Today I noticed a small hill has appeared on the substrate, followed by some bubbles coming out. Few minutes later the hill bursted like a volcano (!), releasing large amounts of gas and a lot of the soil, messing up the entire tank, and leaving a 6” crater.
There was no unusual smell.

What can be the reason for that, and how can i prevent future eruptions?

Please advice, thanks in advance.
 

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I experienced this. Ever hear of a lawn bubble? Sometimes they're called lawn blisters. Usually a pipe bursts and a bubble forms beneath the turf. They're kinda funny until it happens to you, then not so much.

In my case, it was the result of using dry, fluffy soil instead of mud. I flooded my tank, the weight of the water & cap trapped the air in the voids and tada! Tank bubble! Just like you described.

Use chop sticks or a pencil to poke holes in the substrate every inch or so and it'll be fine.

So, wet your dirt until it's good enough to make mud pies, not soupy, more doughy. Then install it and it will decrease those air trapping voids. The big problem with these bubbles is that, as you've found out, is your cap will be breached. In every case of mine of a breached cap there was a bad nitrite spike so test your water and be prepared to do some water changes. Hopefully, that's not a problem for you.
 
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You have a very organic substrate that's very active. As bacteria decompose (eat) the organic matter they generate tannins, CO2 and other gases. The gases collect into a big bubble and explode.

I would poke the substrate as advised to release the gases before they explode. Eventually, the bacteria will have chewed up all the "fast food" and the soil will settle down.
 

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If your tank is two months old, it shouldn't be more than a couple weeks.

By poking the soil, you will introduce oxygenated water into the substrate. That will speed up decomposition of the organic matter. (For bacteria, aerobic respiration is much more efficient than anaerobic respiration and fermentation.) The added oxygen will speed up the decomposition process.
 
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