When should you keep a lamp for fish tank on?

It is generally recommended to keep a lamp for a fish tank on for around 10-12 hours a day, during the daytime. This is because fish need a consistent light cycle in order to stay healthy and active, just as humans do. Leaving the lamp on for longer periods of time can stress the fish and disrupt their natural sleep cycle, while also encouraging the growth of algae. It's important to find a balance between providing enough light for the fish to see and rest comfortably, while also preventing the tank from becoming too bright or encouraging algae growth.