I'd never heard of OATA before - I've just looked them up. From what their site says, they're just a trade association for fish dealers etc? Certainly nothing to suggest they're the "RSPCA for fish". Even then, you do realise the RSPCA have NO powers, NO legal standing and NO right to ask questions, enter premises, seize neglected livestock or anything but launch a private (civil) case against a person the same way you or I could?
What I'm saying is, forget the RSPCA and follow the advice I and others gave earlier in the thread. Speak to your Environmental Health department about it. Without a pet shop license, they won't be abusing any fish, period! I'm sure the EH team will have a suitable 'word' after you air your concerns.