Well, from my experiences of an outside tank and using heaters,
it really just depends on the brand and the wattage.
For example, I just setup my 55 gallon a week ago, and I'm
using an Aqueon 300watt heater. (able to heat 75-90gal aquariums)
I don't know why but, I think the thermometer on it is messed up
or something, it is 4 degrees (F) more than you set it. I've
even tried returning it. I set mines up at 68F (lowest) and it
goes all the way up to 80F during the day, and 70F at night.
I then tried putting in my Eheim 250Watt heater and I set it
at 73F. It still goes up to 80F during the day (weird), but
keeps it at 73F, no lower.
I also have a 29 gallon outside which also uses an Aqueon
300 watt heater and it dips to 70F at night, but stays at
72-75F during the day. (partially shaded)
I set the heater at 73F. I would say get another heater?