Sure this happens but if a store builds a business model around this they are done. There aren't enough people like you to keep an LFS in business. The new customer who purchased the same tank will likely spend multiple thousands of dollars over the next year in equipment consumables and livestock. The service customer who purchased the tank will lead to even more sales because in addition to the dry goods and live stock. They will pay even more in service fees.
The experienced hobbyist has their own dry goods they like so while they may get some from you, some you won't stock so they will order online. The experienced hobbyist is also trying to keep costs down so they will buy bulk additives online and use eCommerce rewards programs. The experienced hobbyist has specific livestock they want and unless you have that exact crazy named frag you won't get the sale. O and if their buddy has some they might trade for it. Pretty hard to compete against free. Not to mention if you have a healthy tank you won't come in as much. New Hobbyists come in to solve algae problems, disease problems, nutrient problems. Experienced hobbyists don't come in. Their tank looks good. If it is doing well enough they will stop buying livestock since it is full.
Experienced hobbyists are not very lucrative customers for the LFS.
Why would someone who bought a tank online be less likely to go to an LFS to buy coral and fish? In any case - I believe that most if not all LFS can sign up with a credit company that allows 0 percent financing.

