Hi Nando7...first of all welcome to 3reef and best of luck with your tank!
As far as temperature is concerned, YES, you want to have a heater as you want to keep your water as close to 76-80degrees IMO! You want to avoid fluxuations as well as mentioned above to reduce the sresses associated with it and the possibility of ich developing in fish, and bleaching in coral!
You want to avoid a high temperature of over 87degrees which is the natural thermal tolerance for corals. Higher temps will cause an increase of CO2 and a reduction in DO(dissolved Oxygen)! A constant temperature kept in your tank will offer you a greater amount of success over the long run! _________ 125gal.w/Mag9.5 return(dual megaflow)>Mag7 pump Aqua Cev180skimmer.Wave2k Hamilton Reefstar(2)250watthqi(mh)pend.a Yellow, Naso Tang Red Lip Blenny Percula Clown Demoiselles Niger Trigger F. Wrasses Ceriantharia Orn.Shrimp and Stars Hermits Queen Conch asst. snails> Stars Zoos shrooms Montipora Brains Gorgonians Favia Turbinaria(large+small polyp) Acropora Xenia Tridacna (CroceaMaximaSquamosa) Leathers <35+75gal.reef tank as well> 
"IF THE PHONE DOESN'T RING...IT'S ME"  jb  | 
Heating Question 
,yes get a heater in mate . too much heat fluctuation will cause some issues with your critters..im in australia which we use celsuis so i keep mine between 24- 26 degrees all the time , its the summer at the moment it goes up to 27.5 - 28 . degrees . hope this helps
