Most shop display tanks are about 2ft long x 12inches wide x 12 inches high. If you make them wider it gives more water volume. A lot of shops have 2 tier stands but you can make 3 or 4 tier stands to reduce wasted space.
Quarantine tanks that importers use are about 3ft long x 2ft wide x 18inches high. This is due to the number of fish they bring in, usually 5000-10000 neons, 2000-5000 male guppies, etc. Obviously you don't need to import that many fish at a time but you have to pay quarantine to inspect every shipment and there are associated fees with them checking the fish during the holding period. So the more fish you bring in, the lower the price is per fish when it comes to fees.
eg: you bring in 1000 fish total. Quarantine fees are $1000 per shipment. This means each fish costs you $1 for fees just to keep them in holding tanks for a month before you can sell them. That price has to be added to the price of the fish, plus the price of importing the fish into your country, plus you need to cover the cost of feeding, water changes, heating, medications and losing any fish.
If you bring in 10000 fish. Quarantine fees are $1000. This means each fish cost 10cents for fees.
So the more fish you can bring in, the cheaper the fees are per fish.
In addition to this, you pay an yearly fee to have a quarantine room. This fee is paid annually regardless of it you bring in 1 shipment of fish, or 12 shipments. So this fee has to be added to the price of the fish too.
Obviously quarantine fees and charges vary from country to country and some places don't have any importation requirements, but most countries have some.
Due to the fees and charges most importers try to do shipments as often as possible. Using the quarantine tanks as holding facilities while you sell stock is a waste of money. It is best to use them for holding new fish for quarantine purposes, then move them into shop tanks to sell while new fish are brought in and quarantined.
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The other option is to buy fish from an importer rather than bring in your own stuff. It saves a lot of time, money and hassle but the fish cost more. However, by selling online you reduce overheads and save money that way.
eg: neons cost 5 cents to buy from an Asian fish farm. After all the expenses of importing, quarantine, etc, and assuming you don't lose them to neon disease, they might cost you 40cents each. If you lose half of them to disease it costs you 80 cents per fish. If you buy neons from an importer you might pay 60 cents - $1 per fish but it doesn't cost you anything to bring them in, and if you have dead fish in the bag, most importers will credit you for the dead fish.
An advantage of importing fish is you can bring in unusual stuff that most places don't carry and you can supply the rare or unusual fish as well as common stuff. However, rare and uncommon don't always sell quickly so you might have a tank of fish sitting around for 6 months before any sell. But if you get a good reputation for unusual fishes, then they sell faster.