As some one who has worked in the industry and been to many stores the number one thing to keep in mind is know what you are. Are you an online coral vendor, a Maintenance business, a full line store? All the time stores make decisions that don't make sense given what there business model is.
If your goal is maintenance and installation. A small showroom style store is perfect. limited dry goods for standardization between clients, extra effort into display, and the ability to get custom equipment and tanks is important.
If you want to be a coral vendor, my experience is that most market places cannot sustain a coral only vendor for local sales. You need an online presence. This will likely be over 50% of your business.
If you want to be a traditional aquarium store like I grew up with then you need to sell everything someone needs to walk out the door with a tank today. This is the one advantage the traditional store has over the internet. I loved working at the LFS. At the shop I worked at you could walk in and leave with everything you needed to set up a 200 gallon reef tank. More and more I see stores wondering why they can't sell tanks like they used to but they don't have the skimmers, sumps, or lights to pair with the tanks in the store. This represents alot of upfront cost but to be honest most small businesses need at least $100,000 of inventory to start. If you don't have the cash you probably should be doing something else. The LFS I worked at had approximately $200,000 of inventory at any given time. I have seen many stores who couldn't pay their rent if they sold everything in the store.