in the uk the law covers any animal, as far as im aware, and i know it states they must be properly housed and catered for at all times while under your care.
for example i would call in the RSPCA if someone had freshwater tropical fish in a tank that had no filter, heater or both. its simply neglect of an animal under your care. of course the RSPCA officer is far more likely to get a conviction if it were a fluffy being tortured or badly cared for but in their eyes they are all animals and they would try and get the harshest punishment possible. unfortunately the punishments are not hard enough.
http://www.independent.co.uk/news/u...ection-from-laws-on-animal-rights-527157.html
as for illegal fish, i wouldnt have a clue becuase all banned fish here are coldwater and i know nothing about coldwater fish.