Light is the input that drives everything else in a jar. Too little and the plants fade. Too much and the algae takes over. The useful skill is not finding the perfect lamp; it is reading the jar and adjusting one variable at a time.
Start with duration
Most low-light jar plants do well with eight to ten hours of light a day. Six hours is enough for Java moss and anubias. Ten hours suits hornwort and water sprite. More than twelve hours is almost always a recipe for algae, especially in a new jar.
Consistency matters more than the exact number. A timer is the cheapest piece of equipment you can add. It removes the daily decision and prevents the “I forgot to turn it off” algae bloom.
Intensity
A jar does not need a powerful light. A desk lamp with a cool-white LED bulb, placed 20–30 cm away, is usually enough. If the light is too close, the top of the jar gets much more light than the bottom, and algae grows on the glass near the surface.
Signs of too much light:
- Green film on the glass within a few days of cleaning.
- Green water.
- Plants growing pale and leggy rather than compact.
- Algae on the leaves of slow growers such as anubias.
Signs of too little light:
- Plants losing colour or turning yellow.
- Stems growing long and thin with large gaps between leaves.
- Java moss looking brown and sparse.
- Very slow growth over several weeks.
Window light
A bright window can be enough, but direct sun is dangerous in a small volume. It heats the water quickly and drives algae. A north-facing window or a spot a metre back from a south-facing window is safer.
If you use window light, watch the seasonal change. A jar that is fine in winter may get too much sun in summer as the sun moves. A simple paper diffuser or a move to a shaded shelf can solve it.
Colour temperature
For freshwater plants, anything between about 6,500 K and 8,000 K looks natural and supports growth. Warm white bulbs (around 3,000 K) make the jar look amber and can favour some algae. Red-heavy “plant” lights encourage compact growth but are not necessary for the plants in this guide.
If you are using a household LED, a neutral or cool white is a reasonable default. Do not overthink the spectrum; duration and intensity matter more.
The daily rhythm
Plants and animals expect a night. A jar that is lit 24 hours a day never lets the plants switch to respiration and never gives the animals a rest. Keep a dark period of at least eight hours.
If the jar is in a room that is dark during the day and bright at night, the light schedule can be inverted without harm, as long as it is consistent.
Signs to adjust
Algae on the glass every week: shorten the light period by one hour, or reduce intensity. Do not clean and increase light at the same time.
Plants yellowing, slow growth: check that the light is actually reaching them. Floating plants can shade the lower half of a jar. Increase the period by one hour, or move the lamp closer.
Water green within days: cut the light to six hours for two weeks, stop feeding, and do a partial water change.
Plants pearling (bubbles on leaves) in the afternoon: a sign of active photosynthesis. It is not a problem; it usually means the light and plant mass are well matched.
A simple schedule
For a first jar:
- 8 hours of light a day.
- A desk lamp or a bright, indirect window.
- A timer, or a fixed daily routine.
- No direct sun.
- Adjust one variable at a time, and wait a week before changing anything else.
The Pondlet simulation uses hours and intensity as separate settings for the same reason. In a real jar, the two are tangled together; on a screen, you can see which one you are changing.