70 degrees is fine for nighttime. These guys are temperate
turtles and live in cold areas. I have never used a water
heater for them.

As far as the basking area. It should be about 85-90 degrees
and you will need to get a thermometer and check it. At this
close range you will probably only need about a 25 watt bulb.
Put your thermometer on the basking area, wait about 10 minutes
and see what it is. You will need to make adjustments based
on that temp. You may even need to raise the heat source
somewhat. The most important thing is to make sure there
is a range of temps so the turtles can thermoregulate. They
will not thrive if their environment is kept too warm without
being able to cool off.

Hope this helps
Linda