As shown in the diagram the proposed automatic fish aquarium light optimizer circuit consists of just a couple of transistors as the active components, wherein the NPN device is configured as a common collector while the other PNP as an inverter.
During day time the solar panel produces the specified amount of light conversion supplying the common collector stage with the required amount of voltage. The NPN transistor base is restricted with a maximum of 12 V with the aid of the connected zener which in turn ensures that the potential across the connected red, blue, green, white LEDs never exceeds this value regardless of the solar panel peak voltage levels.
During dusk when the solar panel light begins deteriorating, the LEDs also experience a proportionately diminishing voltage conditions simulating a proportionately dimming effect in their illumination levels, corresponding to the sunlight....until it's almost dark when these LEDs completely shut off.
In the meantime, as long as the solar panel voltage maintains an optimal voltage the PNP is forced to stay shut off, however as the sun begins to set, the potential at the base of the PNP device begins falling and when it falls below the 9 V mark, prompts the connected blue LEDs to brighten up slowly until these become fully lit after dusk.
The process gets reversed at daybreak, and the cycle keeps repeating simulating a day/night cycle light effect inside the fish aquarium
The 9 V at the emitter of the PNP may be derived from any standard 9 V AC/DC adapter or simply from a cell phone charger unit.