[BMW Rider]

The presence of the ammonia and nitrite suggest that the tank is going through a new cycle or there was just too large of an increase in the bio load. There should be zero ammonia and nitrite for the fish to survive. The best thing to do, would be to save the remaining fish by moving them to an established tank. That might mean returning them to a LFS if you don't have a tank available. Then wait for the cycle to finish so that there is zero ammonia and nitrite. Ideally the nitrate should also be very low or zero also. You can reduce the nitrate with water changes, but not before the cycle is finished, that will just prolong it. As for the chemical additives, they are largely useless in my experience at speeding up the cycle process. Nothing is going to work like time.

Then you can begin to restock - 1 fish at a time preferably to allow the nitrifying bacteria population time to grow to meet the bio load. When you do start to restock, select your fish carefully. Several angels in a tank will fight for dominance. Usually only one species of dwarf angel can be kept in all but very large tanks. Some fish should not be kept together at all. Some should be last additions because of their aggressive natures. Some fish will not be suitable due to the tanks size, even at 90 gallons, a 10" tang is way too big for the tank. Tangs need big tanks, a smaller species would be better suited to your tank. A general rule of thumb for stocking is 1" per 5-10 gallons (like any rule of thumb though, it is hardly an exact science). That would allow you to have an eventual maximum bioload of 9 -18 inches of fish.

You should also invest in a quarantine tank for all new fish. Bring them home, put them in QT and keep them there until you are sure they are not bringing along unwanted diseases. Usually four weeks is reccomended, but less time may be accaptable for healthy fish. This also allows them to get over all the stress of capture and shipping before having to fight for a spot in their new home. A good QT setup would be a 10 or 20 gallon starter tank like the pakage setups sold by most pet stores. Toss the decorations and gravel and keep the tank bare. A few short pieces of PVC pipe are all that is need to provide hiding spots for the fish. You will need to have this tank set up and cycled as well before you can use it.

One of the best investment you can make to start off right is in a few good reference books. It'll give you something to do while you wait for the cycle to finish.