Hi & welcome
When setting up a new tank,the bit the pet shops dont tell you is that your filter will need to cycle before it is safe to add fish,chemicals like stress zyme & stress coat aren't much use really.
When fish are put into a new tank with a new filter,they pee,poo,breath and waste food etc all produce ammonia which is very toxic to fish,in time the ammonia converts to nitrite which is also toxic to fish eventually produces nitrates which is less harmful and is removed with weekly waterchanges.This is all called the Nitrogen cycle.
The filter needs to cycle on average this takes 4-6 weeks.
I would suggest at this moment do a large waterchange with water conditioner such has aquasafe or prime etc,theres loads of choices out there,try to match the temperature of the water also.This will make the fish more comfortable has you probably have ammonia and/or nitrite in the water slowly poisoning them.
Daily waterchanges will help until you can test your water.
You will also need a liquid test kit to test for ammonia,nitrite,nitrate & ph,these are readily available,the most common one is API master liquid test kit,which can be bought on ebay for around £20.
Having this kit will help you to determine how safe your water is for the fish.
You will do good in reading this
Beginners section it has fab info on fish in cycle and other very useful info.
Good luck and let us know how you get on
