Sealed or unsealed?
This question can also be framed as “Do you want to play on hard mode?” While it might not be immediately apparent to newcomers, this decision affects how you will set up your jarrarium. In fact, depending on your goal, this decision is probably already made for you.
If you are simply trying to have a miniature ecosystem in your house you’ll probably want the jar open. Real ecosystems have access to fresh air, after all. You might even opt for some plants or sticks that emerge from the surface of the water.
If you are going for a tranquil aquatic scene – a piece of art – you’ll also want it unsealed. This will inevitably require some maintenance: trimming, removing dead matter, perhaps treating for algae. You will require access to the jar.
There are only a couple of scenarios in which you would want a sealed jar. Are you seeking a challenge? Can you find the solution scientists have been searching for these past few centuries? If not, how close can you get? I’ve had sealed jars last longer than a year. Can you beat my high score, or your friends’? Or perhaps, like my childhood self, you are simply curious about what life is like in there. There’s no better way to find out.
The advantages of having an unsealed jar are numerous. First, and most important, it allows you to occasionally change the water. While the goal of a jarrarium is to create a very low-maintenance aquarium, water changes will be necessary at some point to remove the toxic chemicals that accumulate as waste products from all forms of life. Another vital advantage of unsealed jars is that it gives allows gas exchange at the water’s surface. Gas exchange is, in this instance, characterized by oxygen and carbon dioxide moving from air to water and vice versa. In a closed jar, the system is restricted to the gases it started with, but an open jar allows the system to self-balance somewhat.
Why not a sealed system?
With that said, sealed jars are the pinnacle of this hobby. Many people are drawn to jarrariums and the like because of a desire to attempt to create a self-sustaining ecosystem. Whether inspired by laziness or the thrill of challenge, self-sustaining ecosystems are a common end-result for aquarists.
What makes sealed jars difficult? Simply, the inability to adjust the ecosystem as it changes. Make no mistake – they will change. Maybe the snails you stocked the jar with will lay eggs and their population will boom. They might eat all your plants. Your plants could die for a number of other reasons: light, temperature, lacking nutrients or carbon dioxide. Or your plants could grow too fast and use up all the resources! Without the ability to adjust chemical levels through water changes or trim plants, you sacrifice the longevity of the jar. The most common end for a jarrarium is algae slowly taking over, choking out other plants. As the light and available nutrients are hoarded by algae, eventually any animals within succumb to the green scourge.
Sorry if that sounds morbid.
If you’re not too demoralized, continue on to learn about salt vs freshwater jarrariums.