The short answer
A lid isn’t strictly mandatory, but it’s strongly recommended for most tanks. It does three useful jobs: it stops fish jumping out, it cuts evaporation (so you top up less and the water stays warmer), and it keeps dust, curious pets and stray objects out. For open-top aquascapes people do go without — but they accept the trade-offs.
What a lid actually does for you
- Prevents escapes: startled fish jump, and a fish on the floor rarely survives. A lid is the only reliable safeguard.
- Reduces evaporation: an open tank loses water (and heat) constantly. A cover slows that dramatically, so you top up less often and your heater works less.
- Keeps heat in: less evaporation means less heat loss, helping your heater hold a steady temperature.
- Keeps things out: dust, aerosols, and inquisitive cats.
When people go open-top
Aquascapers often skip the lid because it lets them:
- Use bright pendant lighting without glass in the way.
- Grow plants that emerge above the waterline.
- Avoid condensation and glass reflections for photos.
If you go open-top, keep fish that don’t jump, lower the water line a couple of centimetres, top up evaporation regularly, and expect to run a slightly larger heater.
The practical verdict
For a beginner or anyone keeping active surface fish, use a lid — it’s cheap insurance against the most avoidable loss in the hobby. Most tanks come with a hood or glass lid included.
If evaporation is heating-related, see do I need a heater and the heaters hub. For lighting an open-top scape, browse the lighting hub.