Op is no longer unaware of the law. Its no longer a valid excuse. If you know a law is being broken and hide that fact than you are just as guilty as the one commuting the act. I would not service the account.
I would more say the OP is aware there is some sort of a law, however the species and the scope they are not aware of. However since it's hypothetical it doesn't matter much since it’s not going to happen.
It I had a servicing company I would have a section of the contract or some written portion that would have the owner initial a statement or similar content to “There are no illegal animals in the system(s) to be serviced”.
I am assuming if a person saw a clearly endangered animal like a green sea turtle they would report it, however the laws are clearly confusing and species identification are really outside the scope of what a person cleaning an aquarium is expected to act on.