Live bearers do best with a M:F ratio of 1:2-3 or all female. Males may get teritorial. High nutriet levels and high light promote algae growth. Unless your water is very green, you shouldn't be having low O2 issues when the lights go off- algae will create O2 when the lights are on, but will use it at night- adding an airstone or two, or otherwise increasing your surface agitation will help that.
As far as cleaning your tank, a scrubbing the algae and vacuming the gravel with a siphon should be all you need to do. Any more may set your cycle back.
If new fish are dying, chances are it's a water quality issue- fish that lived in the water as it got worse and worse will do better than new introductions that didn't adjust as the water changed.
As far as cleaning your tank, a scrubbing the algae and vacuming the gravel with a siphon should be all you need to do. Any more may set your cycle back.
If new fish are dying, chances are it's a water quality issue- fish that lived in the water as it got worse and worse will do better than new introductions that didn't adjust as the water changed.