If I want to start an aquarium I will need fish stock to get it going. How can I assure that fish stock for my aquarium are coming from sustainable sources (e.g. being farmed) as opposed to coming from over harvesting of natural ecosystems? Are there clear standards of labeling or do I just have to depend upon the knowledge of the store I buy them from?
The first thing you want to do is read up on the varieties of fish. Some fish like goldfish, are purely domesticated. Some other fish pose rare problems (neon tetras for example). There are other fish where wild caught fish are possibly being sustainably harvested but tank-bred fish are more popular (cardinal tetras).
There are no clear standards in labelling, but you don't have to trust the knowledge either. If you start by researching the varieties, you can get some pretty clear information pretty quickly.
In the end you are going to want to have your own knowledge so you know what questions to ask. Expect to take multiple trips to the pet store with research in between.
Buy them from the breeder. If you can't do that, you are pretty likely to be buying fish that are harvested in deeply unsustainable ways.