Nice tank and fishes
If you’re using indirect light,. try using the “AO only” option, Ambient Occlusion is less to calculate and there’s little visible difference and a bit time difference,. but the use of Indirect light, depends on the objects around the scene which light would bounce back from, like walls and ceilings,.. so if for example, If you have a glass on a table, then the table will bounce most of the light up, and the glass surfaces allow most of the light to pass through, so… depending on what’s in you’re scene, it can be a waste of time to use indirect light,.. I usually try scenes with a much lower quality render, before I think about using any indirect, light or things like Caustics.
Also,.. try a render with “Gamma correction” enabled, and set it to 1.8 or 2.0 which should lighten the whole scene.