The short answer: a lightly stocked, well-planted jar can run without a filter or an air pump. The longer answer depends on how much life you put in it.
This guide explains what the equipment does, so you can decide for your own jar instead of following a rule.
What a filter actually does
A filter does three things:
- Moves water. Circulation brings oxygen-rich water to the surfaces where bacteria live and prevents dead spots.
- Provides surface area for nitrifying bacteria, which convert ammonia into nitrite and then nitrate.
- Removes particles if it contains mechanical media such as sponge or floss.
In a conventional aquarium, the filter is the main home for the bacteria that process waste. In a jar, the glass, the gravel, the plants, and the hardscape all provide surface area. The jar is a diffuse filter. That is why a jar can work without equipment.
When a jar does not need a filter
A jar is a reasonable candidate for a filterless setup when:
- The volume is at least 2–4 litres.
- Plant mass is generous and actively growing.
- The animals are small and few: shrimp, snails, daphnia.
- You feed lightly, or not at all.
- The water surface is wide relative to the volume.
This is the classic shrimp jar. The plants make oxygen during the day, the surface exchanges gases, and the shrimp produce little waste. A filter would add flow that the jar does not need and could even stress tiny animals.
When a jar needs more help
Consider a small filter or an air pump if:
- The jar is heavily stocked or contains fish.
- The water surface is small (a tall bottle, a narrow neck).
- The jar is warm, because warm water holds less oxygen.
- You see animals gathering at the surface or gasping.
- The water has little movement and develops a film on the surface.
A small air-driven sponge filter is the usual choice. It provides gentle circulation and biological surface area without creating a strong current. A simple air stone can also help with surface movement, though it does not add filtration.
Oxygen at night
Plants produce oxygen during the day and consume it at night. In a heavily planted jar with few animals, the night-time drop is small. In a jar with many animals and less plant mass, oxygen can fall low before dawn. This is one reason to keep stocking light and to avoid feeding in the evening.
If you want to measure it, an oxygen test kit is more useful than guessing. A reading near saturation in the afternoon that falls sharply overnight is a sign the jar is overstocked or underplanted.
Water movement and surface area
Gas exchange happens at the water surface. A wide, open surface is the cheapest oxygen equipment you can have. A lid reduces evaporation but also reduces exchange; if you use one, leave a gap.
A small air pump with an air stone can be run on a timer at night, when oxygen is lowest. This is a common compromise for jars that are slightly overstocked but not ready for a full filter.
What about a heater?
Most shrimp and the plants in this guide tolerate room temperature. A heater is useful if the room drops below about 18 °C for long periods, or if the jar sits near a cold window. Small heaters are hard to fit in a jar and can cook a small volume if they malfunction, so use a thermostat and check it.
A jar near a radiator or in direct sun has the opposite problem. Temperature swings are more dangerous than a steady cool temperature.
The honest recommendation
For a first jar: no filter, no air pump, no heater. Use a 2–4 litre jar with a wide mouth, plant it heavily, stock it with two or three shrimp and one snail, and keep the light steady. That setup is simple enough to learn from.
Add equipment only when you have a specific problem it solves: low oxygen at night, a jar with fish, or a jar that is too warm. Equipment is not a substitute for plant mass and restraint.
If you want to experiment with the trade-off, the Pondlet simulation lets you add animals and watch oxygen fall, then add plants and light and watch it recover. It is easier to see the relationship on a screen than in a jar you are afraid to disturb.