Just caught up on this thread. Growing up I had all kinds of fish, African Clawed Frogs, Firebelly Toads/Newts, Turtles, and then snakes that I'd catch. I was literally in the pet store 2 times a week buying something, or begging my parents to buy something. I'd swing by once a week to buy guppies for the newt and rosies for the frogs and turtles. It was just awesome, so I can completely see the market you are targeting here even if you aren't dealing with reptiles/amphibians.
It seems like in an industry such as yourself holding down costs is essential. I would avoid contracts at all costs until you can project a return on investment through sample advertising and research. My brother in law opened up a Taekwondo school in September in a city of 90,000. I made his website and had him listed on Google[posted quickly and didn't get to finish this]. He signed a 4 month contract for some flyer at $400 a month, which he thought would be a better ROI because of the type of city it is (not very tech savvy). Bottom line is the flyer has been running 3 months and he has had 0 signups, where as he's had over 7 signups through Google alone.
One trick you can do is that the 2010 census data was released and you could locate similar cities with your demographics and call another local fish store in another area. Ask them questions, network with them, tell them you are there to answer questions as well. Your businesses aren't competing and you should work together as much as possible to engage customers better.
I project you seeing between 77 - 113 receipts per month in a moderate scenario. Have a set goal per transaction and develop techniques to get customers coming back. You may even offer filter cleanings, tank cleanings, and maintenance. This will get customers back in the store to not only pay for the service, but to buy their fish food and other items while there. Depending on your town, how big is it? Maybe you offer some sort of direct shipping to the customer or drop off to certain areas. Can't serve the entire city? You need to gather as much information about the customer as you can. If someone comes in for a purchase, try to get their telephone number, email address, and name. The highest ROI in internet marketing is email marketing if your subscribers are built this way.
Using Square is smart, keep your payment system simple and dynamic. Sure the fee is insane on a grand scale, but like Lyrical said, this will allow you to focus on getting customers in the door. Keep all payment options open; cash, credit, check
Whole prices are best, keep the operations of your business as simple as possible. This also includes loyalty and membership ideas. Those efforts will not drive customers into your store. People who look at buying a fish don't do it on a loyalty card. You do not want to create extra work for yourself in areas that will take time away from really matters.
If you get a lot of drive by traffic, keep your logo/name/etc VERY simple and easy to interpret. You need to catch peoples eyes and have it instantly click in their heads without thinking. I'd recommend using the word fish in your name if you aren't.
For online marketing, keep your website simple and make sure you are listed on Google local places, bing places, and yelp. Ask each customer who makes a purchase if they would go share their opinion of their shopping experience on google or yelp. I'd say pick one or the other and rotate to keep it simple. Don't sink too much time into exotic fish seo if your city isn't more than 500k+. If you are the only one in town selling exotic or high end fish then you will automatically win that market by default.
I do recommend pay-per-click aka google adwords or adwords express for you. Google fish store [city] and see who is there. If 2 people or less are there (or if the ads are irrelevant) you are golden and won't be spending hardly anything for advertising. Make sure your operating in terms of cpa, not impressions (not sure why they was mentioned in regards to search marketing)
If you are able to get fish food at $3.50 and people on amazon are selling it at $7 or $8. Use that to leverage your business. Purchase 20+ units at wholesale and send them to amazon to be fulfilled. You would be bringing in between $1-2 per sale for doing nothing. If you are afraid of losing customers to online sales, then open up your own storefront via shopify and try to retain customers who need convenience. I can tell you right now for shipping you will pay:
$30 per month for stamps.com
$5.50 per shipmentish anywhere in the US
You may want to leverage some of those opportunities online if it helps your business during down time. On the other hand, do the flyers and phone book listings, but stay the hell away from contracts.