Angelfish Sitting At Bottom Of Tank

Sorry to read about this.

I also changed the filter inserts a week ago and had a bit of cloudiness but that went away.

Almost certainly this is where the problem lies.  Think of the filter media (filter inserts) as the fishes' life-support machine; all the beneficial bacteria reside in these that process the fishes' waste, converting the toxic ammonia to less-toxic nitrates.  By changing the filter inserts, you'll have lost those beneficial bacteria and inadvertently started a fish-in cycle.  You don't mention an ammonia reading but it's very likely there is ammonia presence and this is what is causing the problem.
I would strongly advise (i) getting an ammonia test kit, so that you can measure ammonia/nitrite/nitrates daily or twice-daily for the next 6 weeks or so until the cycle is complete and (ii) doing water changes each day to keep ammonia and nitrite at 0 and nitrates no higher than 20 above your tap water nitrates level - the exact proportion requiring changed may be anything from as little as 10% once a day to a couple at 50% per day but dependent on your test results. If you can also get a bottle of Tetra Safe Start, then add it to the two filters - no guarantee it will work but it has the most likelihood of any beneficial bacteria products to help the fish-in cycle that you've found yourself in. NB. Do a water change this evening - don't wait for the test kit.
In future, no need to change these filter pads - just gently squeeze them in old tank water.

Separately, in order for medication to work, make sure any carbon is removed - carbon is only ever needed after medication to get rid of the remnants of it in tank water.

Great.

The API Quickstart might help but might not - definitely worth a try, though, as you already have it and given the circumstances. Some filter brands recommend changing the pads every so many months, although usually one at a time to avoid loss of beneficial bacteria. In reality, you don't need to change them at all, ever - unless they disintegrate completely which is unlikely. The carbon is usually black - from my recollection of the u2, there were two sponges and a thinner black carbon pad that went in between them, but I'll check the specifics of the u3 to see if it's the same...

Edited to add: Have just found this video https:///www.youtube.com/watch?v=eDjxwCTzvV4 and the bit you don't need (but can keep in a cupboard and use only for a week or so if you ever need to rid the tank of medication) is the bit referred to as the carbon pads at 03:10 and 03:55.
 
Best of luck - hope the angelfish start to recuperate soon!