Why Does the Gravel in My Sandbed Clump?

CastAway · Oct 18, 2014

My tank has been running seven months. I have a very shallow sand bed, maybe 3/4" at the deepest points. It's the CaribSea Seaflor Super Reef. Tonight I stirred the gravel a bit and noticed it had solidified into large hard pieces. I've had high alk for a time, in the 12-13 range, and only recently lowered it. I suspect I have had high Ca too, with no real demand. So what exactly caused my gravel to do this. Is it irreversible? If I manually break it back up, will it release anything?

Reefing Madness · Oct 19, 2014

Mine does this in spots due to the critters that live in it.

billwill · Oct 19, 2014

It is because of the high alk and calcium isn't it? It's not gravel; its crushed calcium carbonate.

Randy Holmes-Farley · Oct 19, 2014

The high alkalinity and calcium certainly can contribute.

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

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.

CastAway · Oct 19, 2014

Excellent info!

So does one simply break it up, so as to have the sand bed intended?

If so, are there any risks in doing so?

Randy Holmes-Farley · Oct 19, 2014

If you can break it up, that's fine. You may want to replace it, or ignore it.

Sand stirring creatures seem to prevent it from happening as much.

CastAway · Oct 19, 2014

Perhaps there's one of those white sand sifting starfish in my future. Thanks guys!

Marksreef · Dec 12, 2014

Hey Randy

LOL, I asked you this question about 10 years ago and think I got the same answer. When I tore down that tank the crush coral was hard as a rock and wide spread. Kind of like a bare bottom with the C.C. look.
Well I have a 7 month old tank now and I'm getting it again so I searched to see if maybe something more of an explanation might be out and NOPE. :squigglemouth:
I thing I might go with the fine sand and suck the C.C. out... Like then and now. I thought the B-ionic had something to do with it.

Randy Holmes-Farley · Dec 12, 2014

Marksreef said:
LOL, I asked you this question about 10 years ago and think I got the same answer. When I tore down that tank the crush coral was hard as a rock and wide spread. Kind of like a bare bottom with the C.C. look.
Well I have a 7 month old tank now and I'm getting it again so I searched to see if maybe something more of an explanation might be out and NOPE. :squigglemouth:
I thing I might go with the fine sand and suck the C.C. out... Like then and now. I thought the B-ionic had something to do with it.

Might be the same word for word.

I copy and paste some of my standard answers, sometimes with a little updating or personalizing.