I do not believe that they are selling 100% chloroquine and I noted this before ever hearing about their dosing schedule. I remember reading years ago about the LD50 (The dose where 50% of fish will die) was not that far from the 10mg/l or 40mg/gal dose that is recommended. The range between recommended dose and LD was narrower for quinine. I will see if I can find it again. I believe there is something "fishy" here.
Storage of chloroquine was studied because it is used in high quantities for malaria prevention. It is degraded by UV light. They are just wrong. Especially at the pH of seawater.
http://www.ncbi.nlm.nih.gov/pubmed/7948188
Next, the attorney for the animal pharmacy I use told me that cholorquine requires a script from a vet. Selling it without one could cause the pharmacist to lose his/her license. Now if you are not a pharmacist and just a retail business - if you get caught selling something that you shouldn't all they will do is make you stop and/or fine you. But again it makes me suspect about dealings with anyone who sells it that way. There is also one source that mislabels their bottles to avoid the issue and makes it look like they are selling a dose lower than requires script. Maybe I was given wrong info but until I hear different, it requires a script for any quantity over 250 mg.
Finally, Dosing sub-therapeutic levels of any anti-microbial will cause the rapid development of resistance. I totally disagree with the Pet store dosing recommendations listed above and pray it is not being done wholesale. The way these fish are maintained in closed systems before export and in the wholesale units - will cause a great medication to be completely worthless in no time.
Penicillin killed Staph aureus probably near 100% in the 30s when first used in patients. In the past two years the major labs stopped testing for susceptibilities to PCN because penicillin sensitive Staph aureus is so seldom seen. We have never treated mass numbers of people with sub-therapeutic levels either. Much of the advice given above is just bad or wrong.
If you are to use chloroquine, which in my opinion is a great med - please follow the recommended dosing guidelines of 10mg/l or 40mg/gal. Dose for at least 14 days and remove the chemical when the fish have finished being treated. Buy it from a reputable and ethical source and make sure it is the real stuff and has not been cut with filler. Anything else is bad medicine.
Rant over