1/4" acrylic lids for the main 3 openings, back couple inches of the tank are open as they are odd sizes and didn't come with lids. I considered this in a previous post on this thread about how the back filtration is laid out. I have a set of extra lids for the main openings, I thought about cutting them into make shift lids for the back, but really didn't think a few inches of openness would be enough to drop that much temp in that amount of water. We have more cold weather and potential snow over the next few days, I may saran wrap the back areas and see if anything changes temp wise.
Yeah I would tend to agree with ya I wouldn't think just a few inches open would make that much of a difference. I can't say for sure though, worth a try through covering it and seeing what happens.