well there's one possibility which I hope is not the case..... but may be. Living in poor water conditions for the last 10 years could have caused some damage to him, things like ammonia poisoning sometimes don't kill a fish, but cause permanent damage to their respiratory systems. I had a fish like this, we had some tank problems when he was young, he survived them but was always a bit behind from that point on, he never grew to full size and died a few years earlier than he should have done. So while I hope this isn't the case, it would be unfair of me not to warn you that it may be he is damaged by the previous problems and will have a shorter life span than usually expected.
Now that being said, this could be wrong and he may make a full recovery, so lets hope and aim for that! The laying on the bottom is also a common symptom of bacterial infections so I would start with an anti-bac medication immediatley. The test kits can be a touch tricky to read, when you take a test, hold the test tube against the white card of the colour chart and hold it up to a light, that will get the best result, even then thought they aren't always clear. If you suspect ammonia though you should be doing daily water changes, just remember to top up the anti-bac meds afterwards. Don't be too concerned over the media dying off, it's fairly unlikley that this will happen, and if it did I'm not too far away from you so I could sort you out with some mature media within a couple of days.
If there is ammonia it'll probably just be a mini cycle, sometimes it happens when you move filters around despite your best endevours.
Torn fins should heal fairly easily providing there is no underlying problem, if they don't start to heal up with the anti bac treatment and water changes then you can try melafix, it's a natural treatment (based on tea tree oil) which aids recovery when fish get torn fins or injuries.
Ok, I will try the anti bac meds, I have some interpet anit bac No 9, is that ok? I was using it for the other fish if you remember, thanks for the offer of the filter media, that would be great, if it dies off, I would like to get this sorted as i go away on 8th July.
It is difficult reading the tests, but due to no nitrite, I am pretty sure it has no ammonia, my son got me thinking, as I was sure it was yellow, his eyesight is not brilliant though!!!