I'm in agreement with bscman. Water test kits are vital to keeping a healthy fish tank, especially when first starting in the hobby or a new aquarium.
As far as your filter, I can't find a lot of information on it, but the little I have found suggest that it is suitable for 12 gallons. Usually these ratings overestimate a filter's capacity. But even at 12 gallons, your aquarium (roughly 34 gallons) is almost three times that size. Goldfish are very messy eaters and easy to overfeed, as they tend to always act hungry.
My best guess without knowing your exact water parameters would be that you have a buildup of toxins that is negatively impacting the health of your fish, and that is why you are seeing them laying on the bottom. I would recommend getting a much more capable filter for your aquarium (if you would like help selecting an appropriate filter let me know) and increasing the volume of your water changes to 50% per week. I would also make sure that you are feeding very sparingly. You may already be doing that, but I would say no more than your fish can completely consume in 1 minute. Again, I'm taking shots in the dark without knowing the exact water parameters in your tank. However, given the information you have supplied I feel fairly confident that is your problem. I hope that helps.
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