mylar baloons work well also, but correct me if im wrong, white paint is suposed to work the best?
Yes and no
White paint absorbs the least light of anything; thereby making it the best color choice on a light fixture.
However.... A chrome or aluminum reflective surface "reflects" more light than it absorbs.
White paint isn't really reflecting the light, but rather just not absorbing it so there is more to be directed into the tank. The reflective surfaces (mirrors or best though) actually change the direction of the light and put it in a different direction (in this case "hopefully" the plants).
This is why headlights have reflective surfaces behind them and not white paint.
Now there could be arguments in the other direction, but IMO (and that is what it is, I am no expert in light) it is better to reflect the light.
My logic is based mostly off of the headlight example. Headlights, most flashlights, spot lights, etc. It is also based off the fact that if someone takes something reflective and shines the reflection into your eyes it hurts a lot more than someone holding something white and trying to do the same
PS - I have no problem with any other opinions showing up in this thread, I would love to hear them