Nitrites should be ZERO, not 1. I think you're not giving it time to cycle. That and a bacterial colony will adjust over time when the fish load goes up or down (as more or less food dictates how many bacteria there can be). Declaring a tank "cycled" and then tossing in the maximum number of fish it can handle all at once can result in temporary ammonia and nitrite spikes which can kill fish.
Also be aware that silver dollars like to be in schools of 6 or more (just like other tetras), and get really really big. Not the best fish for a 30.
I would stop buying fish for the moment, and figure out just what you want to put in this tank - then ask here about things like compatibility issues, how many individuals of different species make them comfortable, and adult size of the fish in question. In general, a 30 makes a good tank for a few different species of smaller pretty fish. Rather than getting 3 of this and 2 of that, often the fish will be happier (and the tank will look better) with larger schools of each species. I've kept tanks like that with a few dwarf gouramis or peaceful dwarf cichlids, a whole bunch of one kind of small tetra (like all neons or all brass tetras, or black neons, or cardinals), and a small group of smaller cory cats - just 3 species in the whole aquarium, but larger groups.
and no, the light doesn't need to be on 24/7 - this won't make the fish comfortable and will promote algae growth. I usually go with 10-12 hours a day.