hi Felix, welcome to this monie-draining pothole.
IMHO, u'll be better off ridding aiptasias by injecting kalk paste with a syringe onto the aiptasias. melt instantly. do a search in SRC, u will find out more.
no hard and fast rulez as to how many hours of lights u shld switch on. depending on what corals u keep, etc. a general guideline will be anything from 8 to 10hrs. Coralline algae primarily flourishes under actinic (420 - 450Nm) lights and additives such as magnesium, calcium, etc.
IMHO, it is good to get a Ca Reactor to supplement for ur corals' needs. but for a start, weekly/fortnightly check of water parameters wld be good. add additives accordingly to replenish the necessary trace elements. alternatively, u can do regular water changes.
IMHO, it does not matters much. beneficial bacteria colonise on rocks, sandbed, etc.
again, no hard and fast rulez here. get a UV which is appropriate for ur tank size. adjust accordingly to instructions given. btw, i was and still am never a believer of UV. so my info on this might be wrong.
external filter as in cannister? get good and reliable ones. e.g. ehiem. cannister filter works well, but regular maintenance is needed. my ehiem 2026 never seems to hv given me problems. esp heat. temp of ard 27 degrees will be good enuff for most corals to thrive.
hope i hv helped.