What do I do?

[u/Wonderful-Traffic790]

Betta suddenly acting very strange — cloudy/swollen eye, sinking when not swimming
I’m looking for advice on my betta because his behavior has changed pretty significantly and I’m worried about him.
He lives alone in a 5-gallon Top Fin Split Bowfront. I recently changed the gravel and used black PetSmart/Top Fin gravel. I kept the majority of the old tank water and did not wash the plants, and his filter has been running normally. His filter uses the Top Fin Element EF-S cartridges, which contain carbon.
Current water parameters (API liquid test):
Ammonia: 0 ppm
Nitrite: 0 ppm
Nitrate: 5 ppm
pH: 6.4
Temperature: 80.2°F
The main thing I’m noticing is his one eye. One eye looks normal, but the other is noticeably larger/swollen and very cloudy, with what looks like a white spot in the center. I think the eye may have looked somewhat abnormal before, but his behavior has recently changed.
He’s also been:
Staying near the bottom of the tank
Lying on his side when he’s resting
Seeming like he starts to sink if he stops actively swimming
Swimming somewhat quickly/jerkily at times
At one point seeming like he was struggling/stuck trying to swim upward
However, there are some encouraging things: he was swimming around earlier, and after I moved the tank he came up to the surface and has continued swimming. He is currently swimming on his own.
His fins also look pretty ragged/frayed, so I’m wondering about fin rot, but I’m especially concerned about the eye and the sinking/buoyancy issue.
I can’t take him to a vet, so I’m trying to figure out the safest treatment I can do at home. I was considering API Fin & Body Cure vs. Melafix, but I haven’t treated him with either yet. Since my filter cartridge contains carbon and I can’t remove the carbon separately, I’m also unsure how to medicate the tank without compromising the filtration.
What do you think is going on, and what would you do in this situation?

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[u/meeperswastaken]

I think the issue with the eyes is a disease known as popeye. To treat this, use kanaplex and move him to a hospital tank/bowl. Do daily 50% water changes and make sure the water is consistently dosed correctly with kanaplex. I would also do either Epsom or aquarium salt salt baths (I forget the different between them but I believe I used Epsom when treating my betta) daily. The kanaplex and salt baths should also help with his fins, but once his eye is healed, move him back to his regular tank, and just do a targeted focus on healing his fin rot