My attempt at translation (not a linguist...)
- (used like a conjunction) guides the explanation of the cause/origin or a reason. The reason is..., because ....
"I was troubled for an answer. Actually, I've never even thought about such things before."
- (used like a conjunction) indicates the subject
"By 'talk', did you mean about that thing?"
It seems the question you linked is discussing the second sense (subject) while here you seem to have the first (reason/cause). Another dictionary linked on the same page says it's a variation of
というわけは (~ is the reason)
Back to the sentence,
目と鼻の先 is an idiom meaning "face to face", "very close", "just around the corner" and so on, so I imagine something like:
However, because the aquarium where Touka(?) remained was "right under one's nose" as they say, it wasn't a particularly good situation in regards to [my] mental state.
Since というのは is not simply indicating the subject here, you can't just replace it by は, you'd probably need to modify the sentence to show the connection in another way.