Your combination of hobby experience and job in car sales is a great start.
Working at a distributor for 7 years at the time when the chain stores were wiping out the independants, I saw the subtle differences that made some stores survive and others fail. I was also trained to be the district manager overseeing the aquatics dept.in 15 stores.
1) Dont be afraid to take a loss on some items. No mater what you pay for them, sell your tanks 55 gallons and below for less than the other stores in your market. Your resulting volume will get you to the next price break level, and you'll generate lasting business. Make up your own starter kits of different price ranges (good, better, best)for the tanks that do not include things they only need after they buy livestock (ie. food). You dont nedd to physically put them together, just list the different combinations on sheets inside the tanks. Then to get the items, the customer will look at almost everything shelf in the store. People generally only buy 1 tank every 2 or 3 years, but buy supplies at least once a month forever.
2) Put the big sellers in the back. Make people walk past all the expensive fish to get to the tank of damsels.
3) Draw up a "planogram" for you tanks. People like to buy from full tanks. Your tanks dont have to be full, but they need to look full. Try to have at least two species per tank. Spread out the larger slowing selling fish and mix the fast sellers in with them. Example: Put a few butterflies in the clown tank. They will sell slower than the clowns but that tank will always appear full. 3 3"-5" Clown Triggers and 10 2" Humas, etc.. Of course your fish experience will help in deciding appropriate tank mates. Keep the biggest sellers in the eye level row.
4)Use dark gravel in all tanks, and lighting is no place to skimp. Keep tank decorations minimal, 2 per tank. Paying someone to remove and replace decorations all day will cost you labor $ and impatient customers.
5)Quarantine everything for 2 weeks. Acclimate them after that in tanks connected to the main system but not visible to the public. Customers will insist on buying anything they think just came in, if you refuse they will buy elsewhere, if already acclimated you will never have to say no. Being able to say "I respect you as a customer and feel good about you having him" is priceless.
6) The idea of having a lounge area is great. (thanx JesikaRose) So many store owners say it's a waste of money to have people hanging around, they are wrong. Put some chairs and a coffee table in the back by some great and varied DT's. Put some general aquaria books on the table. It's a good place for high end Product pamphlets. Dont use the plastic displays though, it will look like a sales pitch instead of fun information. Heres what happens, as a sales guy this will sound familiar. The books will create knowledgable customers, if you run a good business, they are the best kind. Envy, you can only read a pamphlet about a product that is fancier than what you have so many times without upgrading. The greed factor; If you sit 3 people down in a relaxed atmosphere all bragging about what they have,they will all want what the other 2 have.
7) Make sure that the sales staff aren't afraid to admit they aren't sure about something. Coming to you or another guy who does know is always the way to go. Respond to them in a way that doesn't make them feel bad about not knowing or they may start guessing.
8) If you put something on sale, make a sign for the front of the store and one that looks the same by the item (same color, shape, etc) I don't know how many times I heard a store owner say something wasn't selling but didnt bother to make a sign.
9) If something isn't moving, put it on an end cap. Items on end caps move on the average of 30% faster that they do on the shelf.
10) You mentioned discount cards. Another related idea is to give people free fish journals. They dont have to be fancy, just type somthing up and have them done at Kinkos. You write down what they bought, tank size, livestock, water test results, etc. Give them a discount whenever they bring the journal in (5% on over $20, 10% on over $50). You'll be able to track their habits, what they do good with, what they didn't.
11) Barely noticable ambient ocean noise is a powerful tool. A little louder than whatever equipment they can hear running.
Well thats some right off the topp of my head, if i remember a particular stores "secret weapon" I'll post it. hope this helps
