Do you fill up a fish tank to the top?

No, it is not recommended to fill a fish tank all the way to the top. Here are a few reasons why:

1. Gas Exchange: Fish need oxygen to breathe, and the surface of the water in the tank is where oxygen exchange takes place. Leaving a small gap between the water level and the top of the tank allows for better oxygen diffusion and prevents the water from becoming stagnant.

2. Jumping Fish: Some fish species are known to jump out of the water, especially when startled or stressed. Leaving a gap at the top prevents them from jumping out of the tank and potentially injuring themselves.

3. Splashing and Evaporation: When the water level is too high, it can cause water to splash out of the tank when the fish move around or during feeding. This can lead to water loss and make the surrounding area wet and slippery. Evaporation also occurs more quickly when the water surface is larger.

4. Filter Efficiency: Many filters in aquariums rely on the water flow and water level to function properly. Having too much water can hinder the filter's efficiency and reduce its effectiveness in cleaning the water.

5. Maintenance: When performing regular maintenance tasks such as water changes and filter cleaning, it is easier and less messy to have some extra space at the top of the tank.

Generally, it is recommended to leave about 1 to 2 inches (2.5 to 5 cm) of space between the water level and the top of the fish tank. This provides sufficient oxygen exchange, reduces the risk of fish jumping out, minimizes splashing and evaporation, and allows for easy maintenance.