It's funny, but the deeper we go into the world of natural, botanical-style aquariums and the associated arcane topics that go with them, the more and more we find that we need to be better acquainted with the "basics" of aquarium keeping. And the underlying theme that I keep going back to is how fishes are intimately tied to their environment. It's important to consider this when creating our aquariums- and equally important to grasp this idea when creating aquariums and managing them.
Lately, we've received a number of inquiries from customers wondering if it's necessary to use aeration in a botanical-style aquarium. And, interestingly, on first thought, I wanted to simply say, "Well, yeah- of course!" However, the idea of "aeration" takes on a number of associated topics...and the primary, overarching concept here is how oxygen gets to fishes...or more properly, how they extract oxygen from their aquatic environment.
First off, let's think for a bit about how fish take on oxygen.
It's important to know that, even though water (H2O) is part oxygen, it is bonded with hydrogen – essentially locked together, thus making the oxygen inseparable. Fish and aquatic animals cannot "split" oxygen from water (H2O) or other oxygen-containing compounds. It needs to come from outside of the aquarium...from the surface, entering the aquatic environment via a process called gas exchange.
Essentially what happens during gas exchange is that CO2 from the water is "swapped-out" for atmospheric oxygen. And, as a side note- the larger the surface area your aquarium has, the greater the opportunity for oxygen exchange there is.
Since fishes live in water, they need to pump water through their gills, an energy-consuming process which is aided by a vast network of filament-like structures called "lamellae", which are some efficient, that the fishes can extract the majority of the oxygen from the water which passes through them (like, almost 80% of it- that's pretty damn efficient, huh?). And yeah, Anabantoids have that extra advantage of the "Labrynith organ" to help them breath atmospheric oxygen as well, a big advantage in the stagnant pools of water they come from in Nature.
Now, that's all well and good; however, because fishes live in water (duh), the bulk of them are highly dependent upon how much oxygen is available in the environment in which they live. Of course, this varies because of many factors, like temperature, water depth, salt content (saltwater doesn't retain as much oxygen as fresh water, FYI), etc., so it's always a challenge (although the fishes likely don't think about it) to extract as much oxygen as possible from the water.
And stuff like medications and other additives- or dissolved substances- can cause oxygen levels to decrease in the water, making it more difficult for fishes to extract it from the water ('cause there's less of it available). So, when you see your fishes breathing rapidly, hanging or even gasping at the surface, it's a desperate attempt to extract as much oxygen as possible from the water, at the most oxygen-rich location.
Temperature is important, too- because higher temperature water holds less oxygen than water of a cooler temperature- and consequentially, fishes- more active at higher temperatures- have to obtain more oxygen...See- intimately tied to their environment!
Now, there is way, WAY more to the science behind how fishes breathe and extract oxygen form water than this pitiful 3rd-grade science class-style description- but you get the idea that it's a process- one that shows you how fishes are intimately tied to- and dependent upon- the aquatic environment that we provide them!
So, do you have to aerate the water in your botanical-style aquarium?
Well, it's a great question!
While the water near the surface will absorb oxygen out of the air, without surface agitation, less of it tends to get transferred down to lower depths of the aquarium. It's something to think about.
So, is supplemental aeration necessary?
Necessary? No. Beneficial? Hell, yes.
Well, air bubbles caused by airstones and such do facilitate oxygenation and gas exchange. How? Well, bubbles create surface agitation- water movement on the surface. This lets more oxygen dissolve- and more carbon dioxide to escape. A bubble provides more surface area...letting that carbon dioxide escape-and thus providing an additional location for gas exchange to take place.
So, while it's important and beneficial, aeration in and of itself is not a "100% absolute requirement." You need gas exchange. You need surface area...
And there are a lot of ways to facilitate and support these processes.
I personally love wide, shallow "all-in-one" tanks, with built-in surface overflows, which pull water from the surface, helping to "skim" the air-water boundry, thus better facilitating gas exchange.
In general, tanks with large, unobstructed surface area excel at facilitating gas exchange. They also make it easy for some fishes to "carpet surf", too- so there is a little tradeoff, right?
And you could also employ some live plants, of course! Plants produce oxygen as a by-product of photosynthesis. When illuminated, they consume carbon dioxide and produce oxygen. In the absence of light, fish and plants consume oxygen and produce CO2, but do not produce oxygen. That's part of the reason why you find lower dissolved oxygen levels in tanks with live plants in the early morning.
Why do we always say that it's important not to "overstock" your aquarium with lots of fishes?
To put it bluntly, if you have too many fishes in your aquarium, the oxygen available in the water can be consumed faster than it can be replenished. And that's a problem! Of course, there are other consequences to overstocking, like a buildup of metabolic wastes that may be too great for the bacterial population in the aquarium to assimilate.
If you're working with a large aquarium, you'll consequently have more surface area. Even smaller aquariums with a wide, shallow footprint are better than the same sized tank with a tall, narrow profile.
And that whole thing that I always bring up about adding too many botanicals at one time...overdoing it...starts to make sense, right? It's all about dissolved oxygen- and trying to facilitate as much of it as possible.
When you use a lot of botanical materials, I think some circulation is important, too.
Oxygen consumption by the organisms living on or in the substrate is dependent on the oxygen requirement for decomposition of organic material accumulated on the bottom of the aquarium, and for vital functions of the life forms which live there. The dissolved oxygen content of the water layer directly in contact with the substrate is much lower than that of the upper layers of water.
So, yeah...if you circulate the water well in an aquarium with a lot of material on the substrate, you can help move some dissolved oxygen already in the water to these lower levels of the aquarium, where your benthic population of organisms work and live.
Yes, aquariums which feature deep leaf litter or botanical beds, and the organisms, like fungi, bacteria, and micro crustaceans, which "work" them, benefit from this gross water movement. And, of course, what impacts the organisms at bottom of the food web affects everything above it. An excerpt from one study I encountered on natural leaf litter beds confirms this:
"...these stressor effects acting on the base of the detrital food web are likely to- directly or indirectly- also effect higher trophic levels of stream ecosystems."
Okay, I'm sort of all over the place with this, but the big "tie in" is that gas exchange and facilitating dissolved oxygen are fundamentally important processes in any type of aquarium- but especially so in our highly dynamic botanical-style systems filled with leaves, seed pods, and their associated biotia.
Bottom line- you certainly can run a botanical-style system without supplemental aeration IF you have sufficient surface area, DON'T overstock, and obey the common-sense "best practices" of aquarium husbandry which have guided our hobby for generations. It's why hobbyists in the 1920's were breeding all sorts of fishes. It's why killie keepers have successfully bred hundreds of varieties in shoeboxes, bowls, and small, filterless containers for years.
Personally, I run 90% of my botanical-style aquariums with filtration, providing adequate water movement and gas exchange. I've experimented with filterless, non-aerated systems, too, with success...because I have a basic grasp of these concepts.
Many of you do, too.. It's not some "secret knowledge" that only a select few hobbyists posses.
Rather, it's about common sense, observation, and understanding.
So, open your mind, do a little research, be patient...and..just breathe.
Stay careful. Stay diligent. Stay curious. Stay educated. Stay bold...
And Stay Wet.