I used Discuss buffer to lower the hardness of the water (because the PH wouldn't budge as we have very high GH/KH) and PH Neutraliser to lower the PH, in a 25% water change. It came down and has been steady ever since.
I did a 100% water change yesterday, same PH, AQ salt, Seachem Stress Guard and Prime, and acclimated him over 4 hours.
I just watched him come up to the surface for some fresh air (I do have an airstone in there). He looked like he kind of shot to the surface with all his might and then almost sank back down to the bottom. This isn't the first time I thought it looked like swimming was an effort for him.
Only silk plants and nothing sharp in there. He already likes to sit in the plants, it's quite cute :)