I would never treat a tank (with coppersafe or anything) unless I knew exactly what I was dealing with. As mentioned, some occasional flashing is normal, especially after a water change. Shimmying can be mating behavior.
But it appears that you are adding new fish without quarantining them -- so you are bound to run into trouble sooner rather than later. Flashing can be a sign of ick and other external parasites. I'm assuming your nitrites went up after the medication, but perhaps not. Agree with others -- nitrites are bad, bad, bad and you're treating a .5 level as not a big deal. Well, it is to your fish.
My thoughts:
1. Water changes to bring nitrites down and improve overall water quality.
.5 is deadly..this is a must NOW, or you Will lose stock
2. Test daily in such a new tank and feed sparingly to get your tank fully cycled.
This is also a requirement...
3. Consider adding Dr. Tim's bacterial additive (I've had great luck with this for jump starting a biological filter)
Unless this additive has nitrite eating bacteria it is not a good idea, it will simply turn ammonia into nitrite...incrreasing levels of the toxic stuff we are trying to remove with WC's
4. Watch your fish very carefully for signs of external parasites, i.e. ick and treat accordingly should you find it.
I would hold off on ANY treatments..until the tank cycles there is not a stable enough quality of water. Use carbon to remove ALL medications and get your bactera colonies stabilized...
5. Make sure your water is well aerated and you're using a good quality dechlorinator (such as Prime)
100% agree... Oxygen(aeration) is important to both the fish and the bacterial colonies, aerating the tank and any buckets of new tank water can be helpful. Prime will render toxic ammonia safely enough for the fish, but again it does work as well with nitrite.
Added note, Test the temperature/pH of new water. Changes in temp/pH during medium-large WC's can cause problems...
6. Add stock slowly over time
DO NOT ADD STOCK!!! Until your tank is established again and there are no more spikes NOTHING should be added, chances are you will lose some stock. Adding fish before it cycles will only disrupt what you have accomplished and make this cycle take longer...Meaning more deaths
7. Always quarantine. Just get a small 10 gallon with an HOB filter. This will greatly reduce future heartache.
100% agree, I QT in a 10g with simple HOB at 78* for a number of weeks. You never know what new fish might bring with them.
Good luck