Being that we are making an aquatic ecosystem, you would think that a lot of attention should be paid to the water part of it. Fortunately, water is pretty simple.
How to treat your water
You have two options. The first option is to create neutral water and use it to fill your jar. Neutral water is the same stuff you fill up a fish tank with. You can buy water conditioner online or in a pet store pretty cheap. Mix the tap water and the neutralizer in another container, and then pour the water into your jar after you’ve waited the requisite time. If you don’t want to purchase a neutralizer, you can fill a vessel with tap water and leave it out for a couple days. This will have the same effect, albeit to a lesser degree. For a jarrarium though, you can probably get away with it. The inhabitants of jarrariums are not so sensitive as those found in aquariums.
The second option is to use pond water. Go to your nearest body of freshwater and fill up a bucket. Get water from the edge of pond, rather than the middle. The deep part of the pond will have a slightly different composition of microorganisms than the edge. You want the kind that live in shallow water, because your jar is shallow!
Try to avoid catching clumps algae and other hitchhikers like snails, bugs or fish. What you’re trying to capture is the microorganisms that live in the water column of a pond. It will be a mixture of phytoplankton and zooplankton. They are integral to the health of the pond, and they will be important to your jar as well. Don’t worry if you can’t see anything; these organisms are virtually omnipresent in all water – you’ll get some.
In most cases, you will want some small amount of pond water to start your jar. It helps to establish a healthy ecosystem by introducing elements that won’t be present in tap water: bacteria and microorganisms. If you are going for that clean, elegant artistic jar you can omit the pond water. It will inevitably carry algae that will mar your piece. Simply use neutral water in that case.
Additional notes about water
In your jar, you will want to fill the water up to an inch from the surface. It’s important to leave some space there, whether or not your jar is filled, for gas exchange. Air moves from in the water to out of it, and vice versa. If you didn’t leave space, it could suffocate your plants and animals in the jar. Another reason to leave space is snails. Particularly adventurous gastropods sometimes venture out of the water and into the air. They’re not always smart enough to find their way back, though, so if your jar is unsealed they might wander off and dry up. No one wants to find an empty snail shell on their counter. If you leave space between the water and the rim, they are likely to stay in the jar.
Don’t worry if your water is cloudy or dirty when setting up your jar. This is common. The sediment will settle out of the water within a couple of days and your jar will become clear again. It won’t harm the plants or animals inside as long as it doesn’t last for several consecutive days.
Now that you’ve sorted out the water, you need to cycle it. Here’s all you need to know about cycling.