Sure, I'm happy to elaborate:
(1) In general I think there's not enough to do. It's weird though, because right at the start it's really slow, then in the middle of your first day you get to buy like 4 things. Then it tapers off until you can't buy anything for several days in a row.
(1.5) A related issue is that the game takes too long to progress while you're not doing anything. If you added the ability to skip forward in time, or a fast-forward button, it might not feel as much like there's nothing to do (although the first day would still seem really busy compared to the others).
(2) There's no going back on any decision. It's pretty clear what the fish tank fixes do, and those are pretty much always beneficial. But with the plots I think it's very easy to mistakenly make a suboptimal decision, especially because on first playthrough you don't know that the oxidizer only decreases fish water cost by 1, for example. A clearer description, maybe on the computer buy page, could help this.
(3) Replying to your question about the demand increase: I kind of assumed I was supposed to lose and the game went on forever? If there's a way to win, it's totally fine, it just means it's hard. (well, maybe not hard - I'm not generally very good at games) But if you want to make it hard, I'd suggest you make trial and error less painful, which I think is addressed by my previous items.
(4) I was not aware of a person appearing by the gate. I tried going through the gate a couple times. It never let me through, so I stopped trying. I also basically had no reason to ever leave the computer screen - or at least that's what I thought. If there is a whole game mechanic I'm missing then I apologize because everything I've written here might be completely irrelevant.