Had the same problem
with my tank because it's right next to a window in a basement suite. Thought my Ebo-jager was toast so I unplugged it, steel-wooled the plug & plugged it back in. Then I threw a blanket (actually a flannel "bird-shirt") over the opening in the back & the temperature is now back to normal (77-76 C)
Now I have the right temp. & no extra electricity or equipment was needed.
See if you can find where the heat is being lost & cover it up.
Anthony