"Better to ask forgiveness than permission". In my own personal experience, if you ask an insurance agent if a policy covers x, he will reflexively reply "no it doesn't, that's extra". or "no". they hate special circumstances like the plague. Surely, you know that no one involved in any way with an insurance company is going to EVER say "yea, sure, no problem. it's covered". Just get a policy and then get a tank. Don't volunteer the information, read the fine print in the policy to be sure it doesn't say "doesn't cover aquariums", and don't lie if asked if you'll have a tank in the apartment. I tried to get a rider on my homeowner's to cover coins and jewelry, and it was a pain. The cost of insurance to cover it ended up being more than the cost of off-site storage, the time to talk on the phone to agent was excruciating, I got roped into an office visit to "review my insurance needs" for my house/car/life/health, it was just horrible. I'd rather sit in a dentist's chair than an insurance agent's office.
