Speaking as a chemist who interned in a water testing lab while he was in college many, many, many, many moons ago...
Both food grade and non food grade plastics will leach chemicals into the water. There is absolutely no way to stop this. Water soluble or not, everything has an exceptionally limited water solubility and it will eventually leach from the container into the water the container holds. This process will take a few months.
Can you test for this at home? Absolutely not. There is no way to test for a plethora of plasticizers by a simply titration or any other colorametric test. For this you need highly sophisticated analytical equipment on the order of $30,000 (gc/fid) to $400,000 (gc/ms, gc/ms/ms, lc/ms, or lc/ms/ms). Plus the chemicals and gasses required to run the analytical instrumentation. So you see, it's a little cost prohibitive ;)
What to do? Buy cheap 2.5 gallon or 5 gallon plastic containers and replace them about every three months.
However, the question that still needs to be asked is are the plasticizers that leach into the water toxic to fish? Well, everything, even water, is toxic at high enough levels (if a hooman drinks enough water, we will throw our electrolytic balance off and die --- no joke).
So, the question then becomes, are the plasticizers soluble enough in water to become toxic to fish, inverts, and/or coral? Unfortunately, for that I do not have an answer...
