Clumpy Substrate

[g6gang]

One day when gravel vacing my tank I notice a 3" clump (in the front corner) where the substrate kind of hardened up. I chopped it up and haven't noticed anything since. Today I noticed that some substrate under a rock has also hardened (I thought it was my pistol shrimp burrows, but he has not gone to that side of the tank). I think the only reason i noticed this was I rearranged the rock and it seems like, the substrate was hardened underneath it (almost to the shape of the rock).

My substrate is from Natures Ocean and my rock was from reefCleaners base rock.

Tank (80G Shallow) is about 6 months old and params have been pretty stable (weekly checks):

dKH: 8-10
ph: 7.8 - 8.3
Calc: 400-450
Mag: 1200-1300

So I am wondering what would cause this? Should I be concerned? Seems to me like the rock may be the culprit? Otherwise in spot checking the rest of the substrate it seems ok (also have pistol shrimp, some nassarius snails, plenty of pods, coris wrasse)

 

[g6gang]

Other things I forgot to mention that may be important is:

Salt Mix - Tropic Marin Pro
Dose: Two-Part Seachem Fusion (20 ml both every like 3 days)

Running GFO/GAC in reactor (cyano prob since started tank)

 

[Randy Holmes-Farley Verified Community Expert]

Exactly why this happens isn't known, but it is not uncommon and has happened in my tank.

Here's a post of mine from 2005 that pretty much sums up what is still known about it:

There are two different theories about how sand beds solidify, and I am not sure which is correct (perhaps both). Such solidification most often happens in new aquaria.

The first possible explanation is simply that calcium carbonate is precipitating onto the sand grains, cementing them together. That would be promoted by high calcium, alkalinity, and especially pH (the higher it is, the more likely it is, but many sands are fine at pH values up to pH 8.5), and low magnesium. It would also be promoted by a lack of organisms stirring the sand, and a lack of a good coating of organics, phosphate, magnesium, and bacteria on the sand (for example, on new sand).

The second theory is that bacteria growing on the sand cement it together with organic or inorganic materials. Depending on what exactly was being deposited, that would be promoted by a lack of organisms stirring the sand, as well as the same factors above.