Based on the trends that I see happening right now in the industry which I currently work and am heavily involved in, you have to do it right if you want to make a very profitable living off of it.
This is what I see needing to be done as more specific advice for fish only.
You need to cater to every spectrum, from hardcore hobbyists to fishbowl level. Accept that you should carry fishbowls regardless of how inhumane they are, because a lot of customers will simply not respond to being educated. This depends on your ethics as well as margins of the business. As an employee I try to recommend proper setups for all fish but your time needs to be balanced as well. You can't spend all day trying to get someone to buy a 5G and sponge filter for their betta. You could potentially just not carry bowls and the like but I have seen fishbowls turn a lot of people into actual hobbyists. Very fine line.
Therefore, make sure you carry good, reputable brands but only a few of them. Only carry things that are relevant in the hobby as well as things YOU actually believe in. Mich easier to sell someone something when you believe in it because that comes across to them. Carry few brands and memorize the facts about them. What is the crude protein in this food? You should know that it is 35%, for example. A good ratio in my opinion is 25-33% dry goods, and 66-75% livestock.
Get the store as small and as cheap as feasibly possible depending on your market. Ideally, the owner should be running the shop pretty much by them self with no or very few other employees. Again this depends on your market.
Cleanliness is paramount. Friendliness and knowledge come second to cleanliness.
Have an online presence. Social media is very important for posting specials.
Work in conjunction with local hobby groups. Work out discounts with them. Have booths or actually hold frag swaps, fish auctions etc, taking a cut of course. This idea needs more thinking though.
Stay relevant in the hobby. Just because you were an expert at one point doesn't mean you should stop learning. Gossip is huge in this industry and it will get around that you don't know anything.
Carry products and tanks on hand. Rent a storage bin for behind the store if you need to to store at least common size tanks. It is very frustrating for customers to come in and say they need something larger than a 55 which I will not have on hand and need to order it in. They will not wait and will go to a chain store where they know they will have it on hand. This of course doesn't apply to custom tanks, drilled tanks or even uncommon but standard sizes like a 50 breeder. Personally I believe that a store should have 2.5, 5.5, 10, 15, 20, 20 Long, 23 Long, 29, 40B, 55, 75, 90, 120, 125 and 220 on hand at all time, even if only one of the larger sizes.
Regarding the tanks in store, going back to cleanliness, make sure everything is concealed somehow. Most stores have fronts that surround the tanks, hiding airlines etc. Personally I would plan to paint the bottom and sides of the tanks black and run sponge filters barebottom in all freshwater tanks. Ideally then have bulkheads in all of them to run a water change system by just twisting the bulkheads open. If this is achieved, it really cuts down on need for employees. If you have saltwater, sump it, run UV sterilizers. Skimmers are not worth it. Use substrate in tanks that you need to based on what is going in there.
Take feedback from people and order highly requested items. Hobbyists will drive hours if you have a good reputation and will pay a premium for cool healthy fish. Specific orders are hard to deal with. We have ended up with hundred dollar fish that nobody else was because in the mean time the guy who originally asked for this fish found it somewhere else. Deposits and full payment upfront are ideal but then it puts the onus on you to actually get the fish based on suppliers lists and have it not be DOA. Vary what you get. Handpick when possible. Medicate tanks as holistically as possible.
This is not an easy industry. Be prepared to spend hours at midnight at the airport picking up fish and doing paperwork, driving back, then getting them in the systems and then coming in the next day having lost a large portion of that order. It is discouraging. People are very very demanding and egotistical, expecting you to be able to fix their problems when they don't really know what is going on or what they are doing, or doing improperly and defending their method.
That's all I can think of right now.