40% per day with 6 in a 50 being fed 4 times per day may not be enough. You may want to start with increasing wc before meds
Ammonia o, Nitrites 0, Nitrates approx. 10ppm always (I've never had my nitrates go above 20ppm). I'd increased my water changes to 70% per day as well.
Ryan is right, start with increasing water change percentages but one other thing, Melafix and Pimafix are completely hopeless. Some call them snake oil as they have very little medicinal value in their ingredients. do you have pictures?
I had read a lot of reviews about them curing diseases. What else would you recommend? Furan 2 is available. Would that be any good?