I agree with the previous replies. Sounds like there's been way overkill in water changes and not enough time for the fish to settle and make the tank their home. 10 to 15% water changes more than twice a week for fish that are new into the tank doesn't allow them to establish territory or the necessary bacteria to detoxify the water. And as previously stated about the melafix, it's not an ordinary medication that builds up as a dose. The strenght of the product does break down after awhile though some trace of it continues working at a subcutaneous level, which gets nullified by doing excessive water changes.
Im also trying to clear up what the symptoms are. You say there's white spots on the fins but not on the body. Right? I run into that now and again and it appears to be something like Ich's second cousin in law but doesnt' seem to bother the fish in any way and usually runs it's course once the immune system kicks back into gear. Lots of times fish get sick when introduced into a new tank. That's because they come with lots of little critters attached to them, but most of the time will only get sick when under stress. Usually it is recommended to use a hospital tank to treat disease, but when introducing something new that gets mildly sick almost immediately, I usually call for salt, heat, and leaving it alone where it is, because another move will only increase stress for the animal and weaken it further.
I'm goign to apologize now for being too blunt or straight forward, but PhishLuvr, If your water parameters are normal, your nitrates are not excessive and all the equipment works right, leave your tank alone. The only thing you should be introducing in that tank for a whole 7 days is the test tube for your water sample. If you have an ammonia spike, it's a sign of depleted nitrifying bacteria or excessive feeding or death. If that is the case then you should do two 20% changes in that week. However, if the ammonia is zero, then at the end of 7 days do a 25% water change, and keep up the routine for the next week.
Fishkeeping is all about patience and although you need to be monitoring minor changes very closely, you also need to pace yourself in how you apply the changes and actions or you'll find yourself goign downhill fast with very few satisfying results. I apologize again to anyone if that seems like too much attitude, but still that's my opinions on why those problems are showing up, and that is also how I have dealt with similar in either my own or other close friend's experiences.
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