• A primer on un-filtered aquariums



    Practical notes:
    -The bigger the tank the better, but you can have a nice small unfiltered tank if you are very careful
    -Use the best lighting you can provide without incurring huge costs (two strips that span most or all the length of the tank are ideal)
    -Use a natural substrate, gravel works well but if you want to give it a try use soil (very clean of chemicals of course)
    -Plant as many species of resistant plants as you can while letting each get some light (the more species the more likely several will become established and they would absorb any compounds in the water)
    -Use floating plants, I think floating plants act as my filter, there are several species (again a mix is ideal so you can keep the ones that establish well in your particular settings). You have to keep removing them to let light get to the other plants in the tank, always remove the ones that are looking the worst, at some point all of them will look nice, in which case just pick some and remove them. If you remove most of them be careful to put any snails or eggs that might be on them back into the aquarium.
    -Have tons of fast reproducing snails, ramshorns or pond snails are ideal. They help keep the algae under control and eliminate any type of extra food that might be around
    -Do a nice cycling, I recommend setting the gravel and plants, adding one pleco wafer of 3/4 inch diameter and all the snails, that will give a fast start. If you have an already established tank fill the new tank with water from the old tank. After the snails finish the food add another wafer and do so for a while before adding fish or other invertebrates. If the wafer goes bad leave it there to decompose (unless is a very small tank in which a food pellet might be a better idea)
    -Under-stock your tank. I am an advocate for keeping only species that can breed in captivity, which are also ideal in this case since you can start with only a few individuals and let their population grow. Unless you are never adding food (which I only recommend in very big tanks and for very advanced hobbyists who can really estimate that their tank will produce enough food for all the fish and invertebrates in it) the population will eventually be too big, so keep taking individuals out when it seems necessary.
    -Do not overfeed too much. This is kind of obvious but it is even more relevant in un-filtered aquariums. A bit of overfeeding however won't do any damage as long as you have lots of snails.
    -Unless you feel like its very needed don't do water changes, and always do small ones (max 25%) when you do. If you set up everything alright you should never need to.
    -Never vacuum the substrate. The dirt that accumulates helps maintain a healthy bacterial community to keep things in a nice equilibrium. If there is too much dirt (to a level that you deem a problem) vacuum half of the tank. You can vacuum the other half again a week or two after, but in general avoid vacuuming since this dirt provides a lot of the surface area necessary for the microbial communities.

    Precautions:
    -It is likely, and probable, that you will have green water at some point, do not worry, its quite good to establish small invertebrate communities, the fish don't mind it, and eventually it will go way (specially with the floating plants). If this happens before you add fish I suggest adding some Daphnia to the tank. They'll reproduce fast, clear the green water, and make very good food for the fish once you add them (I actually use this strategy for my grow out tanks, if the fry needs infusoria I add them before the water is fully cleared, that way they move from infusoria to Daphnia as they grow)
    -Most of my experience is with soft water low pH tanks. Although most things apply in any case it might be harder to achieve with hard water high pH tanks since several compounds are more toxic to fish at higher pH. This just means you have to be more careful with the cycling and the water chemistry (having a calcium carbonate substrate helps with buffering)
    -You have to have plants, otherwise you might need to do water changes way more often (from about never to every some time) and will certainly have algae (just algae on the substrate decoration might be enough in some settings)

    A hypothesis on how it works:
    I haven't done the appropriate experiments, nor do I know of data around good enough to support it, but I will try to make sense of this by providing a set of ideas on how I think it works.
    -The carbon cycle can be easily thought of as a good equilibrium between CO2 and O2 exchange at the water interface, while the plants produce more O2 than they consume (they do consume at night, I have never heard of an aquatic plant capable of CAM metabolism, which allows for CO2 fixation at night) on a day's average, and the animals simply consume oxygen liberating CO2. Only rarely have I seen symptoms of a lack of oxygen in the fish in unfiltered tanks, and it was only when the tanks were overstocked (In general is good to keep the tanks understocked). There is also a microbial component in which bacteria consume decaying matter, which for this purpose makes them the same as animals. There might also be some anaerobic bacteria as a part of the cycle, in which case there might also be a release of methane to the atmosphere. That being said I only recommend using fish that hare relatively low oxygen requirements, obviously air breeders make things very easy if you like them.
    -The phosphorous and micro-nutrients cycles are also very simple since all of them get in the water only by the input of food you provide (I normally don't use fertilizers of any sort), and get consumed up by the plants and algae, which might in turn go back to the animals if they are not strict carnivores. There should also be some accumulation in the gravel but I have never had an issue with it. Eventually all of these nutrients go into the plants and since the only maintenance regularly needed for unfiltered tanks (at least as I keep them) is to trim the plants, they go out of the tank as components of their tissues (or as fish tissues if you are breeding and have to, at least at some point, move the new fish to a different tank).
    -The nitrogen cycle is, I think, the harder to explain, basically in a filtered tank you remove a certain amount of nitrogen every time you change the filter media, since a lot of bacteria lives there and there is also some chemical retention. This means in the unfiltered tank you will have that "unknown" amount as an excess. That can be solved in several ways, one of them would be for the plants and algae to absorb more, which is likely (although they might be CO2 limited as most aquarium plants are, so they would not be able to absorb that much). There is clearly a community of microorganisms in the substrate which takes up a certain amount. The fact that the fish don't die, and actually live quite well, means that the nitrogen must be processed in one way or another. I am inclined to think that a certain level of de-nitrification might actually be occurring. In this case some anaerobic microorganisms (mostly bacteria I would guess) are producing nitrogen gas, which simply goes to the atmosphere. I have a few observations of methane production which implies anaerobic conditions, so I think de-nitrification is likely to happen (nitrogen gas has no smell so I could not detect it as the methane, you can see sometimes tiny bubbles coming up randomly from the substrate, if you get close enough you can smell the methane, which is in minimal quantities so it shouldn't be a worry). It happens in the substrate, in the lower layers, where there is enough accumulation of debris to create an anaerobic environment. It should be quite a good thing for the tank, since it would remove some nitrogen. There have been cases in lakes where it has not been a good thing since the accumulated gases came up all at once killing everything on its way, but given the scale of a tank that should never be an issue (if you see a truly big bubble, like more than an inch in diameter, forming in your substrate just don let it become huge, help it out and that would do it).

    Disclaimer: I think of myself as a relatively experienced aquarist (I have kept tanks all my life, although that's not that long) but I am not an authority. I am a biologist and microbiologist and although my research has not been directly on aquatic ecosystems I have worked a lot in ecology of tropical forests and been part of some research groups in aquatic environments. What all of this means is that the information you can gather here is correct to the best of my knowledge, but it might work through entirely different mechanisms, so please be critical when you apply it. I wrote this article because when I introduced myself some people were interested in what I do and I want to start a discussion on it, hoping to help some with what I know and learn a lot from the experience of others.
    Comments 7 Comments
    1. apistodave's Avatar
      apistodave -
      Nice article! Uwe and I have come to the same conclusions as you. Between the drought season and the rainy season, Amazon fish ride a roller coaster and have adapted nicley. Neither of us use anything but sponges for any filtration and I do water changes about like you do, never. I have sort of come to the conclusion that water changes too often, stresses them a bit.
    1. doctorgori's Avatar
      doctorgori -
      Hello there Guest,
      To view this post you must either register and log in. Thank you!
    1. Sergio's Avatar
      Sergio -
      Hello there Guest,
      To view this post you must either register and log in. Thank you!
    1. tysgrandma's Avatar
      tysgrandma -
      Hello there Guest,
      To view this post you must either register and log in. Thank you!
    1. boet's Avatar
      boet -
      Hello there Guest,
      To view this post you must either register and log in. Thank you!
    1. Kalákala Tú's Avatar
      Kalákala Tú -
      Hello there Guest,
      To view this post you must either register and log in. Thank you!
    1. Sergio's Avatar
      Sergio -
      Hello there Guest,
      To view this post you must either register and log in. Thank you!