Nothing to do with chemicals. The reason they keep such a tight control on geneticaly modified organisms is the potential damage they might do if let loose in the environment. Suppose someone modified a piranha that could withstand the cold. Would you want that to escape into our lakes?
While an orange danio may not seem like a danger (and almost certainly isnt), the science is new and the possible damages are too great to risk. So, it is better to place a blanket ban on such things, rather than risk unforseen disasters. Then things can be allowed on a case by case basis. Right now there are no mechanisms to do that, so even safe things like glo-fish are banned. Better safe than sorry.
Whether it is valid or not, that is the reasoning. Personally I am pessimistic and think one day there will be something, like a super weed or pesticide resistant insect, that will escape and cause a lot of damage.
