Unless you have a customized policy with a rider covering your aquarium, it is probably not covered in the policy. Most incidental pet damage, such as damage from a broken aquarium, carpet or blinds ruined by a dog or cat, or any other pet damage resulting from expected ownership is excluded specifically, so check your policy and look for pet and aquarium exclusions.
Renters insurance is meant to insure your personal property against destruction or theft and to cover you against liabilities associated with your rental home. Since pets introduce additional risk for the insurers, they generally require you to purchase a pet rider, a special type of add-on policy that covers you for pet-related damages and liabilities. Pet liabilities can be damages such as biting or digging for a dog, or they could be damaged property belonging to a downstairs neighbor after your aquarium sprang a leak. In both cases, the special policy would save you from a lot of money in out of pocket expenses.
You should also get a quote and several competitive quotes. Use the quote generator on this website, and you will get an automatic comparison quote from leading competitors. You will find all of the tools you need to pick a reliable and affordable insurance company, along with a database of information to help you make the most informed decisions possible. You can also find local agents who can assist you with your quotes.
The best solution is to contact your insurance company and inform them of your desire to get a fish tank. The company agent will know how their policies are worded and can advise you of the best way to proceed. They will not cancel you for having the fish tank, and a 30 gallon tank is not especially large, but they may decline to extend your policy to cover the possible damages and liabilities which could result.