hi me use to bead discus also now i reef tank.
ok first thing first.
1) water plants also takes in O2 at night.
2) u may have too may plant & fish in your tank liao.
3) as you say your fishes vomit?? it SHOULD becos they are lek of O2 at night.
4) too mach water cahnge at 1 time is no good for your fishes also
5) did you run CO2 in your tank???? try to turn it off at night
i may not be right at all just some thing for you to look into Y..