I don't know much about fish oil, but I give my puppy flax seed oil. She gets this supplement daily. It helps to improve their immune system, insures a healthy heart and great for skin & coat or allergies. It doesn't say anything about weight gain, so I'm guessing it does not have that effect. Fish oil seems to have that side effect. My vet recommended it because Teeka has been doing a lot of scratching. She has only been on it a few days so we'll see how it works for the itch! If nothing else it is a good supplement for the whole body. I hope this helps.