A well-planted aquarium will have plenty of oxygen at night for the fish and flora. Angelfish (Pterophyllum scalare) by Tony Terceira

Airstones
Q. I always use airstones to make sure my fish get enough oxygen to breathe. The pet store told me I should not use airstones if I add plants to my tank. A friend of mine said I should use airstones but only at night. Who is right?
Jan Fletcher
Simcoe, Ontario

A. Let me start with two myths and a rule of thumb. Myth: Airstones are needed to add oxygen to an aquarium.

Myth: Airstones should not be used on a planted aquarium because they remove CO2.

Airstones are unlikely to hurt a slow-growing garden with no added CO2, but they are not an especially useful way to add oxygen, anyway. Airstones drive off any CO2 in the water above the level that the water would have from normal exposure to air. So, if you add CO2 and use an airstone, you can drive off all the added CO2.

When I was a kid (around the time steel was invented), and folks had figured out how to take asphalt, putty, glass and steel, and make an aquarium that only sometimes rusted or gave you a jolt of electricity when you touched it, stores always sold you airstones so the fish could breathe.

We have come a long way since then. However, a new version of the airstone myth has arisen: that aquatic gardens should have airstones bubbling away at night, so the fish can breathe. A corollary to this is that nighttime airstones are needed so that the plants can breathe.

Airstones are a relatively cheap, but rather inefficient, way to oxygenate the water. A small, half-decent water pump will serve the purpose better. That’s notwithstanding the fact that a well-planted aquarium will be saturated with oxygen when the lights go out, leaving plenty of O2 to last fish and plants through the night.

Opposing the “airstoners” are those who argue that airstones remove CO2, so you shouldn’t use them. Actually, an airstone (or water pump, for that matter) will tend to make the water have the maximum amount of CO2 it can absorb from the air, which is about 4 or 5 parts per million (ppm). If your tank has less CO2 than that, an airstone will add CO2. If it has more, it will tend to remove CO2 down to 4 to 5 ppm. Actually, the concentration can be slightly higher, due to CO2 coming from fish respiration.

If there is soil under the substrate, anaerobic bacterial activity can generate CO2 (among other things) and raise its level slightly above 4 to 5 ppm. However, the difference between 3 and 5 or 7 ppm is small in absolute terms because we are talking about very small amounts of CO2. So, as far as CO2 is concerned (in a tank without added CO2), an airstone is probably not going to cause any harm and might even help — but only a tiny bit.

Mixing Materials
Q. My fish store has some gravel that is for 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 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 tank 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, 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 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 tank 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 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 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).