When fish stay near the top of the water, it sometimes because they are having trouble breathing. Oxygen/CO2 exchange takes place on the surface of the water. One quick way to get more oxygen into the water for them is to create more surface disturbance, agitating the surface water more. If you're using a filter that returns water into the tank at the surface, you can lower the water level a bit to ensure the water pouring back into the tank is creating greater agitation of the surface. This increases the rate of oxygen/CO2 exchange. You can also add a bubbler or pump that introduces bubbles or flow that in turn creates more surface disruption.
The next step would be to test the water conditions to see where the water is at, in comparison to the water conditions that the fish need.