I noticed the fish taste too when I used my toddlers Horizon DHA milk last night for cereal. I though it might be residue from the bowl I was using but when I was filling my toddler's cup with milk I could smell it and I tasted it again.
I saw a few other comments on their Facebook page but it seems like Horizon is trying to keep it secret and not admitting any problems publicly. They are telling people to private message them instead of saying what is going on.
I called their customer service number to complain and find out the story. Tory said that what is happening in the algal they use for DHA is "oxidizing". To me that sounds like it's spoiling but she would not say those words. She say the milk is fine to drink aside from the smell and taste. I personally do not think everything is going to be OK with this milk. It was obvious she was choosing her words carefully and could only say certain things. When I pressed for more information she had nothing to offer.
She also said that since it's not harmful (as far as they know) that they do not have to recall it from the store shelves. (which they are not doing)
I urge everyone to call them can complain. They will also give you a refund. Also, comment on their Facebook page and try and get this out in the open.
Consumer Hotline:
1-888-494-3020
(Weekdays 8am to 5pm CT
excluding holidays)
https://www.facebook.com/Horizon