It's a tricky question, isn't it? Most people here will probably say that keeping pets is ethically permissible, if you give them the proper care.
But, it is also legally permissible to keep pets, even without giving them the proper care.
Of course, there are laws against gross animal abuse. But what about substandard or inadequate care that doesn't constitute "abuse"?
Many of us here have probably experienced the following scenario: A person posts asking for advice about his tank, maybe asking about what new plant he wants to buy. He lists his stocking, tank size, etc., and a bunch of people here consider these tank conditions to be horribly inadequate and/or downright cruel. All these people people start posting and criticizing the OP's stocking. The OP responds, "Listen, I wasn't asking about your opinions on my stocking! My fish are perfectly happy, so stop telling me what to do!" At a certain point a mod steps in and says, "This is off-topic--stick to the plant question." And then all of us here eventually have to say to ourselves, "I've stated my opinion, and now what the OP does with his fish is his business."
But we all go away feeling sad, because we know that it's the fish that suffer. We have an intuition that, ethically, the OP is wrong.
But how can we ever bring the law in line with our ethical intuitions? It would be impossible to restrict the fish trade (or the pet trade in general) so as to enforce our own high standards on other people. Wouldn't that, at some point, harm business or infringe on the personal freedoms of others?
In the end, the only protection that animals in the pet trade really have is the caring nature of their owners. Which is to say, pets are at the mercy of the individual humans that own them.
As responsible pet owners, the best we can do to bring the pet trade in line with our ethical intuitions is to be as vocal as possible about our own standards, try and educate those around us, and try to boycott or protest inhumane business practices.