For years, my brother has kept an aquarium that seems to be in perfect balance. He rarely, if ever, feeds the algae eater, six elderly zebra danios and neon tetras. He runs no heater or bubbler or filter. He just lets it quietly be, the plants and fish live on in apparently endless harmony, a few snails cruise the walls, and the water stays clear. We thought it was just one of those delightful things that sometimes happens, but when my eight-year-old wanted to get his own aquarium, I did a little reading and it turns out that natural fish tanks like my brother’s are a “thing.” They use a natural biological filter created through beneficial bacteria instead of running an electric filter and they rely on the oxygen of natural plants instead of bubbling oxygen into the tank. Some call them dirted aquaria, some follow the teachings of Diana Walstad (the Walstad Method) and some, like my brother, just come upon the system through time and luck.
No matter which method you use, a natural aquarium is beautiful way to introduce kids to the concepts of ecological balance, photosynthesis, and introductory chemistry. This kind of tank is not for those who want to see a pristine environment – the tank will ideally mimic a pond environment and will have some beneficial sludge on the bottom, some random little snails and worms, and should have a lot of live plants balanced with a fairly small community of fish (one fish per gallon is the rule of thumb). It takes several weeks to get fully set up, but there are lots of rewarding steps along the way. You can spend very little on equipment, and you won’t have to hear all the annoying hums and vibrations of the bubbler and filter, but do prepare to spend quite a bit on plants and you will definitely want to invest in a quality water testing kit that allows you to monitor water pH, ammonia, nitrite, and nitrate levels.
You will need a tank with a lid and, depending on how naturally you want to light it, you’ll need a decent light so that the plants will thrive. Sunlight is wonderful, but it can heat up a tank or cause algae growth, so make sure that you’ll be able to control the amount of light, either artificial or natural, that hits the tank. Larger tanks are said to do better for a natural aquarium because they absorb chemical imbalances more thoroughly, but for a child, a ten gallon tank will be a fine start so that the water changes necessary at the beginning aren’t overwhelming work. We got a cheap 10 gallon tank from Walmart with an LED lit hood that seemed impossibly affordable but has proved to be perfect for us. It came with a filter which we used for a few hours during initial set up, but we could have lived without it. If you are setting up for a classroom or you want to help care for the tank, you might prefer to start with a larger tank. Just make sure that you have plenty of study support – a 20 gallon tank weighs about 200 pounds. If your aquarium will be in a room that stays at least 68 degrees, and you choose your fish carefully, you may not need a heater. Otherwise, a heater will keep your fish more comfortable. For a thermometer, you can get a cheap little stick-on strip to get a sense of temperature.
There is so much advice out there that I’m not going to repeat all the steps for a natural dirted tank set up in detail, but to give you a short sense of how we did it, we set up our tank in an upstairs room that stays fairly warm throughout the winter and which gets glancing sunlight on one side for a few hours each afternoon. We rinsed organic potting mix and then laid it down in the empty tank in a 1-2″ layer, sloping up toward the back. We then arranged about 15 small dried 1″ balls of terra cotta clay evenly over the soil (the clay supposedly supplies extra iron for red plants), covered that with about an inch of coarse sand (fine sand could trap harmful gasses in your substrate) and then a layer of gravel. We set in some rocks and wood (pick these carefully since some will dissolve harmful metals or chemicals into your tank) and gently siphoned a few gallons of water into the tank. Once the gravel was covered with a few inches of water, we planted a lot of plants. Really go for a ton of plants in here for success. We tried many different variety and most have done surprisingly well. Creeping Jenny has been very happy on the sunlit side of our tank. Amazon swords, camomba, and Amazon moss has done better on the side of the tank that gets only the led lid light. Once the plants were settled, we gently filled the rest of the tank. We have well water, so we only had to adjust our pH but you might need to add a dechlorinator. At this point, we used the filter for a few hours to get the worst of the floating dirt out of the tank. We then did 25% water changes for several days. We scooped a bit of the water from my brother’s tank and added it to ours to jump start some positive biology. The water will be quite yellow for the first few weeks from all the tannins seeping out of the dirt. Just keep changing.
After the water color had settled a bit, we got about 20 Malaysian Trumpet Snails from a nice kid on Craigslist. We got them free, but you may need to buy them. These snails like to turn over the substrate and help keep the ecology at the bottom of the tank healthy. They will proliferate, so introduce them knowing that you will likely have them forever.
Keep changing the water – the tank will go through a cycle over the next several weeks in which ammonia will spike, followed by nitrites, followed by nitrates. The ammonia and nitrite are poisonous to your snails and other aquatic life, so you need to keep changing the water to keep them comfortable. Daily water testing is a wonderful little chemical study for kids, and they really enjoy seeing the colors develop, but be sure to supervise since some of the testing solutions are severe skin irritants. Older children might enjoy charting the chemical levels or learning just how ammonia turns into nitrite and then turns into nitrate. As the plants and bacteria take hold, the tank will gain better chemical balance. We can actually see oxygen bubbles forming on the surface of the plants as they turn carbon dioxide into carbon and oxygen. At certain times of the day, several of the plants actually release a slow but steady stream of little bubbles.
Once ammonia is down, probably after a couple of weeks, you might add an amano shrimp as we did. There is a whole amazing world of freshwater shrimp out there – they are charming additions to the tank and help keep the tank clean. We planned to wait to add any more fish until nitrite levels were all the way down to zero, but one day, at about three weeks, we found the tank absolutely filled with thousands of tiny white detritus worms. Detritus worms are beneficial, but this was far too many and we didn’t want them all to grow. So, we changed the water until the nitrites were down to a very low level and got a few lovely fancy guppies who proceeded to eat every single visible worm within 12 hours. They also cleaned up some of the dead leaves on the less successful plants. A week later, the guppies look healthy and relaxed and one of the females looks ready to give birth. The tank remains clear and pretty although we are still doing 10-25% daily water changes as we wait for the nitrite levels to stabilize all the way down to zero. The water changes are a bit of hassle, but really only take a few minutes and if all goes as planned, we’ll have a stable ecosystem within the next couple of weeks.
The whole experience has been a ton of fun so far and the kids have really enjoyed it. We’ll probably add a few more fish in a couple of months, but we’ll leave things to equalize for now. If you try this method of keeping an aquarium, let us know how it turns out.
Update a week later: The guppy did have her babies, but our original plan to let things take a natural course fell apart because the kids were so sad that they were getting gobbled up by the adult fish. I really shouldn’t be surprised! So we rescued a couple of the cuties and have them safely in a little net brooder to grow until they aren’t snack-sized. The tank chemistry has begun to stabilize and we no longer need to change the water daily – here you can see that after a few days of no water changes, the water test kit shows ammonia is .25 ppm, nitrites are less than .25 ppm and nitrates are under 5 ppm. We’ll likely do a water change tomorrow and we expect the ammonia and nitrite levels to stabilize all the way down to 0 within the next week.