Yes Martin, I have a Eclipse 6 with a betta in it and even though it
has a heater it has never turned on because the temp is always between
80=B0 - 83=B0. If it's warm in my office it can go even higher then that.
Like you, I keep the lid open on the warm days. Fortunately, once it
really warms up here, the AC should keep the tank fairly cool (78=B0).