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.
/www.aquatics-online.co.uk/listsecti...retp=322&rets=XJust 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.