my betta and his 1 gallon tank temp

Question
I am worried about my Beta's tank temp. I cannot seem to keep it from fluctuating. It is too small for a heater and I don't know if it's ok to add a little treated warm water to it when it falls below 70F. One of his fins seems to be a bit shredded and I don't know if it this is attributed to the temp. I am a new beta parent and welcome any advice.  

Answer
Dear Stephanie
Hello i am an aussie so i don't really know if it is summer or winter over there so if it is winter over there it is ok to put a little bit is warm water in there but like cold warm water not to warm if that makes sense or your other option is to get a bigger tank and put a heater in that as for the fins being shreaded that would not be related to the  tempreturei dont know what could be causeing that unless there is another fish in there with him
Happy New Year