Mixing sand? Original grade with coarse - bad idea?


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I am upgrading my 55 gallon fish tank which has 30-40 lbs of CaribSea Ocean Direct Caribbean Sand Original Grade Substrate ("Original Grade Substrate is, by weight, less than 1 mm in size (most of that are smaller than 0.66 mm"). I am buying a 75 gallon running system that was for sale locally and has CaribSea Fiji Pink sand Grain size: 0.5-1.5mm. I'm not sure how much sand is in the system, but I would estimate it to be ~1" deep in a 75 gallon, which I think is defined as a shallow sandbed. Would it be a bad idea to mix the two sand beds? I read that coarse sand can create toxic gas pockets in deep sandbeds. How deep is too deep? Would I be ok mixing them if I'm at ~2 inches deep?

The goal is to provide the best substrate for copepods to feed a Green Mandarin Dragonet.

There will also be a sump with chaeto.

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Mixing sand? Original grade with coarse - bad idea?

I use crushed coral as a substrate. I have mass amounts of flow in my aquarium and anything other then crushed coral was always in the water column. Personally I'm happy with crushed coral sand. Multiple reasons. It doesn't fly up into the water column, it's pretty easy to vacuum, and the 3 leopard wrasses don't have any issues diving into it, even the carpet anemone doesn't seem to be bothered by crushed coral. The only down fall is that if I go anymore when a month with out vacuuming the sand bed the crushed coral grows coraline algae and the sand bed turns pink. I like having the sand bed somewhat white.

Michael Hoaster

Registered Seaweedist
Premium Member
Yes, you can mix grains sizes, no problem. Different grain sizes allow for different pore water spaces between the grains, which facilitates diversity of bacteria. This is good for both nitrification and denitrification, which means less Nitrate accumulating in the tank. So, those fine grains are great for deep sand bed filtration.

The larger crushed coral is also quite useful as a top layer. As Duality pointed out, it doesn't get blown around by powerheads, so you don't end up with bare bottom patches. As a further bonus, a half inch or so layer of it on top provides superb refuge for pods. Not only can they move freely amongst the 'boulders', the larger gaps between them accumulate food right where the pods are.

In my tank, I put sugar size sand on the bottom, a medium grain in the middle, and the large crushed coral on top.