Help

[Flippin'thing bit me eye]

Did you say that you put the filter in with all new matierial?
If so then that is what has caused this, your new filter is having to colonise bacteria / cycle, there isnt much you can do really, I wouldnt do too many water changes as you are only delaying the cycle.
When you change filter you should either have it running for a few weeks with your old filter or at least use the old filter media in the new one.
Dont panic it may be ok.

 

[reg2k2]

While it's normal for any planted tank to experience a day-night difference in pH, I've also read a low-KH level + heavily planted tank can result in *wide* swings..not so good. At night, the plants stop giving off oxygen, and join the rest of the tank in putting out CO2 only....hence the possible O2 problem.

I don't have plants, but my suggestion (to start) would be to run an airstone at night, see if it makes a difference.

Test the KH of your tap & tank.

The white haze likely a bacterial bloom, result of a partial die-off, not harmful.
I see no problem w/ carbon in tanks....not recommended for a planted tank, tho (can take up nutrients better left to the plants).