molly & tetra sick

Question
My tank Details:
Size of the tank=20Gallon
All fish in the tank = 1 lyretail molly, 6 neon tetra, 3 scissortails, 1 swordtail, 1 otocinclus
Filtration= tetra whisper, Medium size(change filter in 1-2 month). I have an airstone in the tank.
Water change schedule=  30% per week
tank temp =78 deg F
How old the tank is= 1 year.
Water parameters: Ammonia=0, Nitrites=0, Nitrates=10mg/L ppm, Chlorine=0, Hardness=90GH, PH=6.8, Alkalinity=80(KH)ppm
Food: I feed them once a day. Types of food include= Flakes, Peas, Brine shrimp.

I have molly and neon tetra who are sick from past 4-5 months. Molly has white patches all over her head and some red sores and it is becoming fatter and the red sores are increasing. On one of her gills, I can see some scales. But it is very active and has a very good appetite. The tetras had some tail rot 5 months ago and one of the tetra has become very fat. But all are active and are the first ones to grab the food.
Over the last 5 months, I have given them course of Tetracyline, Melafix-pimafix, fungus clear, Maracyn medicines (one at a time) to treat bacterial/fungal infections (dropsy,tailrot,columnaris) that they might have. But none of them were effective on molly as well as swollen tetra.

Please advise me what else I can do. I don't know why they became sick as I change the water on time, water parameter are good too and rest of the fish in the tank are in excellent condition. Please help I don't want the fish to die.
Thanks in advance.

Answer
Hi Pal;

Your water is very good. I wish I could get others to be as diligent about maintenance as you are. Good job!

Sometimes antibiotics in the water just aren't effective because the problem is too deep. I would try feeding a medicated food for bacterial infections. They are available at most fish stores. You can even make your own using tetracycline. Here is a web page that explains about different types;

http://thegab.org/Articles/MedicatedFood.html

At Your Service;
Chris Robbins