He possably will be okay =D Mine NEVER had a heater but the light was on 23 hours a day cause I was given faulse info on my first betta, ended up that he was the longest living I have seen o.o And he lived in 86 all the time ^^ . Yours should be a-Okay inless he's a dual tailed betta, they have a harder time with high temps =D but thats why most are called tropical fish, they feel there best at 82-86 and are okay at lower temps till you get to 76 then it becomes dangerus ^^ if you could keep it at 80-83 he'll be better then ever. just watch for bacteria cause thats also there thriving level. o.o Whys your betta at work btw??