Ouch. Well, it might be old age idk.
Really, ZD need to be kept in groups of 6 or more. I am somewhat amazed that they lived 4 years, the 2 of them. I kept a school species in a group of 2 and it introduced all kinds of diesases (beginner days
Even if they are a pair, I am pretty sure they are too old to breed. And I think ZD are egg layers/scatterers (?), and if that were try, the female wouldn't be fat anytime at all. Eggs everywhere. The fat could be from overfeeding, unless you aren't overfeeding them (how often do you feed them?).
That growth on the male looks like a diesase that one of my school fish that wasn't in a school got. It is
contagious from IME, the fish should be isolated if possible.
The curves, I kinda doubt it but it might be fish tb. That usually effects vertical structure, too.
Just keep an eye on them. The diesase may also be related to water quality. It is always a good idea to test it when your fish don't look/act like normal.
Hope this helps. Hope your fish get better!