I'd like to see the aquarium tag stay as is, applied to all aquarium-related posts. Aquariums are a huge topic that has lots of facets. I know that originally there was a separate Aquarium site on Area51 that got folded into Pets.SE. I agree with that decision, but the fact is that there are lots of tags that could be considered "sub" tags of aquarium. Here are some ideas: freshwater saltwater tropical coldwater brackish reef acrylic. I'm not suggesting that we create all of these tags before we have questions for them, but they would be good tags for the right questions.
I would like to see all aquarium posts have the aquarium tag; this allows aquarists to filter pets.SE for their interests. Then if a question is specific to saltwater or specific to reef aquariums, they can add the appropriate tag to their question.
If you have tags labelled freshwater-aquarium and saltwater-aquarium, people will start to type in "aquarium" into the tag field, see the freshwater-aquarium tag come up, and then select it. Most won't also decide to include the aquarium tag, because it is redundant. They'll think, rightly so, that the aquarium tag isn't needed, because the other tag already says "aquarium". So we'll end up with a bunch of questions tagged freshwater-aquarium, but not aquarium.
You might say, "But if we don't include the word "aquarium" in the freshwater tag, then we'll have a bunch of questions tagged aquarium, but not freshwater." You are right, but in my opinion, if a question is only going to have one tag, the aquarium tag is most important. The general tags are always more important than the specific tags. The purpose of the tags is not to define the question; that is done with the title and body of the question. The purpose of the tags is to help users group and filter questions into useful categories. The best way to do this for aquarists is to have one aquarium tag that will appear on every aquarium-related question. The freshwater and saltwater are just optional bonus tags that can be added to questions later by the aquarium question enthusiasts.