water quality color changes.

[jp80911]

Is there any harm in not using gravel or any substrate in a tank? I was thinking when i set it back up to leave the bottom empty for easier cleaning and better filter efficiency?

gravels or other substrates provides more surface area for good bacteria to grow on so you'll have better bio media than what's int he filter alone. however the down side is not all the solid waste will get pick up by the filter which could harm your water if not properly cleaned. if you have the tank planted then the plants will help take care that issue for you.