There are so many urban legends on here regarding taxes it's not funny.
You would only have to file if you were running it as a business and produced a profit. Even if you were foolish enough to waste your time filing taxes, you would quickly find out that you cannot offset personal income with losses from a business that was not your primary source of income. If you claim losses if you do file as a business, you are only allowed to offset future profits in the business. You will not get a refund or offset your other income by producing losses selling corals.
Furthermore, you can only claim expenses for the percentage of use that is dedicated to the business. So a real simple example would be, you have a 400 gallon fish tank for personal use and 100 gallons for business use. You would only be able to claim 20% of your expenses as a deduction. Do people fool around with this? Yes, but they are breaking the law. What is more likely is that the IRS would disallow all of the expenses.
All of this goes along with classic tax myths like, "I made less money because I was pushed into a higher tax bracket." Um no, you didn't. The marginal income was taxed at a higher rate, not all of it.
The other classic myth is, "well the business doesn't care because it just can just write off the expense from taxes, it doesn't cost them anything." Wrong again. Yes, they may be allowed to deduct an expense, but that only saves them whatever their tax rate is, so they may save $0.40 for every dollar they spend. A business will go broke real quick spending a dollar to save 40 cents.
I am sure some people were joking, but if you get audited by the IRS, you won't be laughing.