Can Anyone Help Me Help My Angel

Hi @Cherry Margot
I'm not experienced with angels as I have not kept them myself, so I'm not sure how much help I can give until some of the more experienced keepers pop in.
Did you quarantine the cardinals prior to adding them to the tank?
Have you seen any aggression from any of the other fish in the tank.
What sort of decor do you have in the tank? Anything the fish could have damaged the fins on?
At the moment I'm thinking infection and/or injury, but will have to get some more information & do some more research.

It is possible that the cardinals carried some disease into your tank.
It is common practice for shops to dose their tanks systems with a variety of medications. This can deal with issues, or can subdue problems which can then crop up when you take the fish to your own unmedicated tank.
We tend to recommend a quarantine period of at leas 4 weeks which gives most things time to show up.

Also, just to check, what is your water hardness? And what temperature are you keeping the tank at?
Obviously your tank is well established and your ammonia/nitrite/nitrate levels look great, so I thought I'd just confirms some of the other parameters too.
Also, if you can get any more pictures of the fish that would be good. Not easy, but can be helpful.

At the moment I would be tempted to do one larger water change a week rather than smaller daily water changes. Or perhaps do 2 x 50% water changes a week. This may be enough to give your angel time to heal itself.
As for medication, I tend to use eSHA 2000 for general infection/injury. It's fine for your filter bacteria and won't have a negative impact on your tank. It's also safe for shrimp (if you have any), but is a problem for snails.

As mentioned yesterday, I've not kept angels, so it would be good to get other keepers advice too.

I have hard water too.
Sometimes it can be a problem with limiting the fish that can be kept, but it can also make your tank very stable.
It may be worth avoiding the medication if you have a lot of snails in the tank. If they all die it is going to cause a lot of problems. As you say - carnage.
If your fish seems ok in himself, still eating well and acting as expected, perhaps lots of water changes while he heals will be fine.
Keep a close eye on him to see if you can work out where the initial damage/infection came from. Monitor interactions with other fish for potential harm.
If anything deteriorates (behavior, appetite, mood) then we may have to reconsider medication. If this is the case it may have to be done in a separate tank/container to ensure the snail problem doesn't arise. We'll cross that bridge if/when we come to it.
Fingers crossed that it is a health issue that will sort itself out over the next few weeks. 

Thank you again.I did wonder about Melafix. I have used it once in the past and although it didn't seem to do.much it certainly didn't seem to hurt anything.
Would it help if I raised the temperature at all?
I'm wondering about the SAEs , he is holding his fins in when they are about, but they've lived together fine so so long. I'm thinking maybe because the fins are limp and damaged they are mistaking them for food
If I need to treat him in a separate container I'm guessing he only needs to stay there while the treatment is actually administered? It's going to be stressful for him poor chap

 My SAEs are greedy chaps and will eat anywhere there is food and always join the surface fray at feeding time even though they get wafers and cucumber!
Thanks for all your advice, much appreciated
Ive just done a big water change and will do another tomorrow

Sorry to hear that the fin is broken, but it's good that the fish is still behaving normally.
Doing some research and there is mention of SAE becoming more territorial as they get older/larger, and also some noting that they will nip at long finned fish.
Has anything changed in the tank recently? Any decor moved? Anything that could have disturbed the SAE and/or caused a change in behavior? It could also just be one of those things that we don't get to the bottom of.
Whether or not your SAE caused the original damage, they now seem to be bothering the angels, so you may need to consider splitting them up, either until the angel fins heal, or as a permanent move.
In the short term it may be possible to insert a large piece of plastic mesh into your tank to split the fish up, it is something I've done in the past, but it very much depends on the layout of your tank.
Also, I don't know if having more caves and hiding places will make the SAE a bit calmer and less likely to nip. Or it could just be the the ragged end of a damaged fin is just too tempting to resist. I think you'll have to monitor the situation quite closely and keep us informed.

Thanks for the update.
It's good to hear that the fish have settled and the angel is still eating well.
I hope that the fins continue to heal.