Add another light. You can have it in the room sort of near the tank, does not have to be directly over the tank.
Make that one on the timer/dimmer. Incandescent, of course.
Set it to start coming on 15 minutes before the tank lights, then it can go off once the tank lights are on.
At night, reverse. It comes on while the tank lights are still on, and when the tank lights go off this light slowly dims, replicating sunset.
If you have a fixture that has several levels, that is, not all the lights come on at once, then set it so the smallest number can come on, then a few more, then all the fixture, over a 15 minute period. Reverse at night.
If all the bulbs are wired to come on at once this won't work.
Manually fix it:
Put a piece of cardboard between the tank and the light.
When the light comes on only one row of bulbs are uncovered to shine into the tank.
Over the next 15 minutes take away the cardboard, uncovering one more row... then one more row... until all the bulbs are exposed.
Keep the tank near enough to a window that the rising and setting sun can be seen. Time the fixture to be on only when the sun is up. That way the sun starts rising, slowly increasing the light, then the tank light comes on. In the late afternoon the tank light goes off during the day, then the setting sun 'dims' the tank.