~Ditto ~
Seems to me that if you have a friend in a petstore it would be far easier to ask him about how to keep fish.
You have gone from a cold water fish to tropical without finding out about the simple basics of how to look after the fish.
If you had whitespot with the coldy then personally I would have emptied out the tank and treated ALL of the items in the tank, Stress coat is not a disinfectant so what good it would do to an empty tank is baffling.
Any water changes do not necessarily require you unplugging the heater unless of course you empty the water to such a degree that the elemental side of the heater is exposed.
Any water you replace in the tank should be of the same temp as the water you are replacing, it should be dechlorinated (something your friend at the LFs can suppply you with).
Fish should be fed small portions, if you still see food at the bottom of the tank 20 minutes later then you are overfeeding, so cut the amount down.
One more comment, you added 4 largish fish very quickly and in my opinion before the filter was mature enough to support them, you may have problems with the existing fish so keep an eye out for a rise in Nitrate, invest in a water test kit to keep ontop of the situation.
No one is flaming you but if you ask for advise be prepared to get a COMPREHENSIVE discussion on the subject you raised. If we see a problem we are going to point out any problems, if you don't like it then talk to your friend.
People learn by their mistakes, we on this forum have all been in your position and gladly taken on board all comments made that is how you learn in life.
