2 months is enough time to cycle a tank, but with your current stocking levels, your tank can't support that many fish, it's just too small. Goldfish are huge waste producers so they need larger tanks to support them and their bio loads. Adding PH down was a bad idea, you should never mess with your tank PH (unless you're a pro and you know what you're doing), it is very hard to regulate. This could have caused damage to your fish, and if you are still adding I would suggest that you stop immediately. Further, if you are adding any other chemicals to your tank you could be throwing off your test kits. Things that "elinimate ammonia" (they don't work), "neutralize ammonia", etc. will throw off your test kits so you won't get accurate readings. I would suggest overall to do a large water change in your tank, 50% of treated tap water (with basic water conditioner). Then continually monitor your tank readings, I think you'll find some of them are off. What other chemicals are you adding to your tank?