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> だが、十香のいる水族館と目と鼻の先 **というのは** 、精神衛生上あまりよろしくなかった。

Context: the protagonist (士道) is dating 十香 and they went to 水族館 together. But
in the process of watching fishes in 水族館, 士道 runs away, pretending to go to
the restroom because he has to date another girl, 折紙. 折紙 offers to eat in a
restaurant, which happens to be close to the 水族館. 士道 worries that they might
run into 十香 if they go to that restaurant.

Question: how would the nuance change if we omit the bold というの by just saying
は?

I found a seemingly related question here.

[What is the difference between というのは or は when used as a topical
marker?](https://japanese.stackexchange.com/q/76909/30454)
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question
particle-は
How is というのは different from は?
Although it may seem possible to grammatically replace that というのは with は, in
fact, not really. The last word 先 (part of the idiom
[目と鼻の先](https://dictionary.goo.ne.jp/word/%E7%9B%AE%E3%81%A8%E9%BC%BB%E3%81%AE%E5%85%88/))
is certainly a noun, but if we take what comes before というのは as a noun phrase,
the part 十香のいる水族館と will fail to parse, because [unlike English prepositions,
Japanese particles alone cannot modify
nouns](https://japanese.stackexchange.com/q/27363/7810).

Instead, it is a clause with the last copula (だ) omitted, so that ～水族館と can be
an argument of predicate. Omission of the dictionary form だ(/である/です) is more
normal a practice in speech and literature than otherwise.

Then というの is just doing its job making the subordinate clause compatible with
particle は. と adopts the subclause to make it a functional adverbial phrase in
the main clause; いう makes it an attributive (≈ adjective); and finally の
convert it to a noun in order to connect with は.

> ［十香のいる水族館と目と鼻の先だ］というのは…  
>  "(the thing/fact/situation) that [(it) is eye-and-nose distance (= a
> stone's throw) from the aquarium where Tōka is] is..."

If you don't want to use というのは here, you can have other options including なのは,
which is direct conjugation of だ into nominalizer の.

> 十香のいる水族館と目と鼻の先なのは…  
>  "to be a stone's throw away from the aquarium where Tōka is is..."
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answer
[Daijirin
says](https://kotobank.jp/word/%E3%81%A8%E8%A8%80%E3%81%86%E3%81%AE%E3%81%AF-579247):

> と言うのは（連語 ）
>
> ①（接続詞的に用いて）原因・理由の説明を導く語。そのわけは。なぜならば。 「私は答えに困った。－、そんな事を考えたこともなかったから」
>
> ②（接続助詞的に用いて）主語を示す。 「話－、そのことですか」

* * *

My attempt at translation (not a linguist...)

>   1. (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."
>
>   2. (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.
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