Mixing Aquarium Substrates

Can I mix large store-bought substrate with smaller sand?

Q. My tropical fish store has some gravel that is for aquatic plants, but it’s expensive. I was hoping I could save some money by mixing it with sand because I like the look of the smaller sand. Will that work?
Wilbur Krantz
Chicago, Illinois

A. Yes, you can mix different materials if you like the way they look; but note that if one material is fine-grained and the other coarse, over time the finer material will tend to settle downward, falling between the grains of the coarser material. Thus, the overall appearance will tend toward that of the coarser material.

The information on aquarium gravel can be confusing, so let’s consider substrates and cut through the mulm.

Myth: If you call it substrate or something else instead of gravel or sand, it sounds more important.

Myth: You need expensive gravel with high cation exchange capacity (CEC) values to grow aquatic plants.

CEC is a biochemical thing that folks talk about, and because they do, they must know a lot about biochemistry and aquatic plants in particular, right? Whatever CEC is, folks will tell you that your substrate should have a lot of it. Yet, one of the best gravels for growing aquatic plants is Flourite, which on the basis of the last figures I saw, is relatively low in CEC, compared to some of the alternatives — or at least it is out of the bag. It’s different after the gravel has been in a planted aquarium for a while and accumulated some mulm.

“Mulm” is the technical term that we aquatic gardeners use for the detritus in our aquariums. “Detritus” sounds sort of nerdy; “rotting remains” sounds downright morbid. “Mulm” has a warm homey sound.

Whatever you call it, aquatic plants generally like the gravel better when it has some. Mulm is organic, contains some nutrients and supports a lot of healthy bacterio-logical activity that in turn supports aquatic plant growth. I personally have spent a small fortune on the popular aquarium substrate Flourite — it works, and I love the way it looks. But aquarists do perfectly well with plain old-fashioned gravel or even pool filter sand. Whatever it is that you use, it becomes richer as the aquarium matures.

With some finer materials, you might discover that when you place a new plant into the substrate, it tends to come back out easily. Once the roots grow, however, the aquatic plant will stay in place. So, you have to take a little extra care when planting in finer-grained material, being careful not to disturb the aquatic plant right after planting. A fine-grain material that I like is onyx sand because of its slate black color. I have also used plain river sand with good results; this can be bought from an aquarium store or collected from a river (either will work if it’s inert in water and well-rinsed).

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