Yes, I did a water change right after. Right now, my fish are doing pretty well right now, except that the nitrite levels are really high. I have 1 zebra danio, 2 male guppies, 2 assorted corydoras, 1 blue platy, and 1 sunset platy.
If you're referring to the 10 gallon you have listed in your signature, no I wouldn't add any more fish as you're at or over your limit. This could be the cause of your problems. Because it looks like you may be overstocked, you should increase the frequency of your water changes. Be sure to use a good water conditioner - Folks here recommend "Prime" because it detoxifies ammonia and nitrites. Typically when water parameters go bad it's because of a water change issue or a water treatment issue. More water changes won't hurt your fish. In fact, they'll probably love it!
I agree with Vin, don't add any more fish to this tank and Prime or AmQuel+ water conditioners can save your fish. How long has this tank been set up? Was it fully cycled or are you cycling with that many fish. If so that could be the problem. Do small (10-20%) water changes and test your ammonia, nitrite and nitrate levels daily.
I'm sorry for the loss of your fish and hope you can get it straightened out so you don't lose more.