As I thought it seems the problem is you did not cycle your tank and then added some high waste producing fish such as pacu and oscar. The weeksi is effected by the ammonia probably because its the smallest but, its not long until the others get sick as well. I think your best bet is to do a large water chance. about 75 percent, probably each day until the ammonia starts reaching zero. You may want to also rehome most of those fish in the meantime. hopefully someone with more experience with this could chime in. But you definitely want to do a large water chance (with conditioner) right away.