Online or at home possibility for a business (only open weekends or select holidays).
That is a really hard way to make a living - to keep a bricks and mortar location for fish/pets and supplies. You will need to be really creative if you want to do that. Please do not think I am trying to knock your dream, one can do anything with enough planning, money and thinking outside of the box.
A few things of immense importance, learn and follow all rules and regulations for your city, state and region. Attention to details with things like licenses, taxes and shipping creatures limitations, insurance, employee taxes, and labor board rules, shortcuts can put you out of business, etc. MAKE sure to always pay your sales, and employment taxes. Have an accountant and don't put off those little pesky things like monthly or quarterly filings/payments. Often you can overcome sales issues and disease, but not the government when it comes to taxes.
I have seen business shut down in a flash!
Rent or building use and electric costs are high, and how many individual sales are needed just to support that - to keep the lights on so to speak.
Larger mass of sales is in importing and selling to sellers. Volume - one sale x 1000 fish instead of 6 fish to one person.
Doing what you love can be a blessed hobby but also a nightmare if a business. One is fun the other is constant worry of being able to pay the bills.
So just keep in mind there is a reason many stores close. Dirty tanks, sickly fish and poor service and workers who don't care or steal your $ or stuff.
But on the other hand there are niche and personal service oriented vendors who are loved and supported and thriving (as far as I can see).