The tank is in my office for work, so it is only a 20 gallon tank. I have to small (2") Reds in there. I can work it out where I can make small water changes each day (mon-fri), think is this best method?
How long has the tank been running? It seems as if it has been cycled but excess food, or something, has caused the ammonia to spike up a lot. Make sure you feed carefully and don't allow any bits of food to escape/ blow about. You then have to clean the substrate with a gravel cleaner everytime you syphon water out. Waste food build up can lead to your ammonia rising like that.
If you have been changing the water 5 times a week this would explain the low nitrates, otherwise you may not be cycled. Doing a 5% water change daily is ideal because of keeping the nitrates low. It will encourage good growth. The problem being they don't have long to grow in the little tank. It would be nice to get them at least a 4ft tank around 40 gallons plus.
People will tell you to get a bigger tank and add another fish to calm fighting down. I am not sure if many people have tried this or are just repeating what others say. Friends of mine have grown 2 Pygo's to pretty much full size from babies. They always lived in the same tank without even a serious fin nip, in fact when I see them they look pristine. I can see how adding more may curb aggression, or at least even it out amongst more. But the only experience I have of it is my friends tank, which has been successful.