The light is important as essjay noted. Few if any of the tetras we keep occur in brightly lit watercourses, quite the opposite, and overhead light is a real issue. I have been able to observe significant changes in the behaviour of many of my characins after thinning out the floating plants, so it demonstrates just how important the lighting is to fish.
I don't see any issue with the parameters or the ammonium (in an acidic pH ammonia is primarily harmless ammonium).
One other thing does occur to me...you had three and added three more, and so far they have not actually "gelled." This is one reason I always advise that with a shoaling species, acquiring all the intended fish at the same time together is advisable. Some species establish hierarchies faster, and with more serious intent behind it, but even with fairly peaceful fish it is still advisable to acquire the entire group at the same time. I only mention this because your observation is the reason for the advice, as it comes up in threads and sometimes I think people assume I am just being silly. But it has relevance. Nothing to worry about with this species, or shouldn't be, but advice for the future.
Add some floating plants, lots, and this should settle out. I would also be careful on additives...the less chemicals into the water the better/healthier the fish, and this includes conditioners. One is sufficient, that deals with chlorine (and chloramine if that is added to your tap water). The Tetra AquaSafe is OK until it is gone, then I would recommend another. EasyBalance is not necessary, and given the claims the manufacturer makes for this product I would not use it. It cannot ever replace water changes [I know you are still doing them, I am just responding to Tetra's claim that it does for six months--absolute nonsense]. Prime is another I am not particularly happy with as it messes with too much and in a closed aquarium any such "messing" is never safe. I highly recommend API's Tap Water Conditioner; it is highly concentrated (even more than Prime) so you use very little, and it doesn't do anything but deal with chlorine/chloramine and heavy metals. When the AquaPlus is gone, look for this one...will they exchange the Prime (if unopened) for the API maybe? If not, use up the Prime, then change to API.