Yes you need to do a water change if you have a ammonia reading as it should be 0,with whitespot you can take the temp up to 30, how do the fish look now.
Just add a teaspoon of salt household salt fine if you don't have aquarium salt, as it helps with parasites and stress.
Just another little one, just add the declolrinated water to the tank , if you buy another med get a gang valve for the pump, as you will need the pump running, as you will be able to alter the flow with a gang valve.
Gang valves are further down the page.
http/www.aquatics-online.co.uk/listsecti...retp=322&rets=X
Does that mean you can turn the pump down now, as aearation will help, just add the black carbon for now , as long as they are not gasping at the top, they should settle down.