I've asked two questions about plants in my aquarium so far, and found that I got very good answers too. On the question "Should I let Echinodorus Tenellus flower?" there was some debate as to whether the question was off-topic or not. I think this is a valid discussion.
Do aquarium plant questions fit under Gardening and Landscaping?
In my opinion, the questions you've posted are all on-topic here.
There is a site for pets, and aquarium keeping is part of that site. But they don't do identification questions, and while I don't think questions about propagating plants would be closed if they were asked there, I think this site would be the better place to ask.
If you're interested in getting more exposure, possibly reaching aquarists who might not normally visit gardening.se, feel free to post a link to your question in the chatroom of pets.se.
I think that if your question was more along the lines of wondering if letting the plant flower would be detrimental to the animals in the tank, then it would be better served at pets.se.
It's a question about propagating a plant, so it's on-topic.
I think the person who answered was making a joke that it's a water plant, so it doesn't belong on a site about land-scaping, because "land" and "water" are opposites.
Since that statement detracts from the answer, I've edited it out. I also cleaned up the comments because with that edit they vary between obsolete and distracting.
For me, I think there are a lot of nearly off-topic posts on this site. However, there are no alternate sites for these yet. The ID posts for wild plants (some may be weeds in landscapes - the poster doesn't know) perhaps should be in a Botany or Plant Taxonomy site (the Biology site really seems like a Biotechnology site to me). But there isn't one. I also see questions about growing things commercially that should probably be in Agriculture, Horticulture, Agronomy, or a related site, but there isn't one (Sustainable Living is related). So it's perfectly fine here. Besides I don't want to go to that many sites to see what's up. Aquatic plants in a home aquarium is definitely in Gardening & Landscaping. Water gardening is a hot topic in gardening now. Also anything about propagation and growth of a plant that is raised by humans should be on-topic too. I think the reason there aren't posts is the unfamiliarity with it. I never grew this plant so didn't chime in - Though I do have aquariums and several water gardens (in containers outside).
