Most landlords have this mentality and I can't say I blame them.. but the OP was told his fish where fine.. and now they are changing their minds. breaking a contract on either side is never a decent good thing to do. As a pet owner is was costly to keep our pets because a lot of landlords are against pets.. which was fine. we found places that allowed pets. I have a feeling the OP would have made moving arrangements when the new owner took over had he been told he would not be allowed to keep his fish or any fish for that matter.
can you imagine 1kgallons flooding your basement? lol yes water damage is very possible and if tanks are improperly cared for even more so.
Just more drive to get your own place and stop paying rent... and start building equity