A tank with fish and no live plants
Fish need a regular day and a dark night, nothing more. The room’s daylight plus a few hours of tank light in the evening, when you are there to watch them, is plenty: 6 to 8 hours is a good day for a fish-only tank. Plastic or silk plants need no light at all, and every extra hour feeds algae on the glass and decor.
Switch the tank light off at night. A tank left lit around the clock gives the fish no rest and grows algae fast.
A planted tank
Live plants need a steady day to grow, but more hours do not make up for a weak light. Start at 6 to 8 hours while new plants settle in and lose their farm-grown leaves. Once they are growing and the glass stays clean, add half an hour a week up to 8 to 10 hours. Stop when algae starts to show, and go back to the last length that kept it away.
A tank with a strong light and injected CO₂ runs the CO₂ from an hour or two before the light comes on until about an hour before it goes off, so the gas is already in the water when the plants start using it.
Algae: shorten the day first
When algae takes off, the day is the first thing to cut: take 1 to 2 hours off and leave it there for two weeks. Light is only half of it; algae grows on light plus spare nutrients, so look at the fertilizer dose and the water changes too. The kind of algae says which is out of balance. Identify the algae · Check your fertilizer dose
Green water after the tank stood in sunlight, or after weeks of long days, clears faster with a few days of the light off completely and a water change. Green water
Put it on a timer
The same hours every day matter more than the exact number. A plug-in timer, or the timer built into many LED lights, keeps the day the same length on weekends and vacations. Lights with a sunrise and sunset fade are gentler on fish than a sudden switch from dark to full brightness.
Some keepers split the day into two blocks with a break of a few hours in the middle. It does no harm, but one block is simpler and is what most planted tanks run on.
Sunlight and night lights
Direct sun through a window adds hours of strong light you cannot switch off, warms the water on summer afternoons and is a common cause of green water. Place the tank out of direct sun; the indirect daylight of a room is fine. Keeping a tank cool in summer
Blue night lights or moonlights are for watching the tank, not for the fish. Keep them dim and off for most of the night.