Did you throw you old filter sponges away?75% water change has just been done, whole tank has been scrubbed down with a brand new sponge. Filter has had a complete clean just now too for the new water. I've added your recommended dose of aquarium salt just now until the treatment arrives (eSHa 2000, eSHa Exit). Thank you for your advice. I will be keeping the thread updated.
If yes, get them back into the filter or you will have ammonia and nitrite problems.
Filter media/ materials should only be replaced if they are falling apart or don't belong in a filter (carbon & zeolite). Sponges last for years and should be squeezed out in a bucket of tank water and the sponge re-used. The bucket of dirty water gets poured on the lawn outside.
The filter media, including sponges, hold beneficial bacteria that keep the ammonia and nitrite levels at 0ppm. If you replace the sponge with a new sponge, you get rid of the good bacteria and the tank will have to cycle again. Cycling is where the beneficial filter bacteria grow to a suitable sized colony to keep the water clean. This normally takes around 4-6 weeks.
You need to put the old sponge back in the filter and monitor the ammonia and nitrite levels for the next few weeks. If you get an ammonia or nitrite reading above 0ppm, or a nitrate reading above 20ppm, do a 75% water change every day until the levels are 0ppm.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.