I assume you use drivers between the PCA9695 and your aquarium lights.
They have PWM frequency limitations. Are you sure they like 1500Hz.
Too high could mean not dimming to very low light levels.
Did you first try the default 200Hz.
Your code is incomplete. Do you use a library for that PCA9685.
I use this one, with an equal dim step lookup table designed for LEDs,
which then makes setting dim level (in 8-bit steps) to 40% easy.
You should have started your own thread.
Can still ask a moderator to split it.
Leo..