Ranchu not moving

Tank
What is the water volume of the tank? - 245 litres, or 64.7 gal
How long has the tank been running? - Recently upgraded to the larger tank on 16 Nov. I monitored the fishes progress and it was all good.
Does it have a filter? - Yes. I use a filter canister.
Does it have a heater? - No. I live in a tropical country. Temperature of my water is usually around 28
What is the water temperature? - Temperature of my water is usually around 28°C or 82.4°F
What is the entire stocking of this tank? (Please list all fish and inverts.) - 2 ranchu, 1 ryukin, 7 platies

Maintenance
How often do you change the water? - Every 2 weeks
How much of the water do you change? - 30% or 1/3 of the tank water
What do you use to treat your water? - SeaChem Prime
Do you vacuum the substrate or just the water? - I vacuum any visible poop on the substrate, after that I siphon out the water.

*Parameters - Very Important
Did you cycle your tank before adding fish? - Yes, used all old filter media from old tank to the new tank.
What do you use to test the water? - API Freshwater Master Test Kit
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.

Ammonia: 0ppm
Nitrite: 0ppm
Nitrate: 0ppm
pH: 7.4

Feeding
How often do you feed your fish? - Once a day
How much do you feed your fish? - Enough for them to finish up within 2-3 minutes, I remove any excess food
What brand of food do you feed your fish? - Hikari Lionhead Fish Food (sinking pellets)
Do you feed frozen or freeze-dried foods? - Yes, Hikari frozen brine shrimp or Hikari frozen blood worms, but only once per week.

Illness & Symptoms
How long have you had this fish? - 9 months
How long ago did you first notice these symptoms? - About 5 days ago
In a few words, can you explain the symptoms? - Ranchu sitting on the bottom of the tank. Only moving to eat.
Have you started any treatment for the illness? - Yes.
Was your fish physically ill or injured upon purchase? - No
How has its behavior and appearance changed, if at all? - Used to be more active. Now it doesn't move.

Explain your emergency situation in detail.
My ranchu sits on the bottom of the tank and doesn't move at all. Video link:

https://imgur.com/a/bC3aQ1B

It still eats when I feed them, so I didn't think too much of it at first. Once in a while, I see a platy picking at the sides. Video link:

https://imgur.com/a/BDBZE1U

I posted it on reddit, and someone say it could be constipation, so I fed them deshelled peas. But it still sits on the tank.
2 days ago, I separated it so I could treat it. I haven't fed it for 2 days now, I'm thinking of feeding it deshelled peas tomorrow. And when I view it from the top, the scale at the sides appear to be protruding and I'm worried it could be onset of dropsy. I treated the water with Seachem KanaPlex. It appears dead/not moving (video link:

https://imgur.com/a/XkRXdGJ

) from time to time. I would like to know if you guys have any other advice.

 

I like to change 50% of the water every week to ensure no nitrate or ammonia buildup. You clearly don’t have this issue, but just for the future that might be a good precaution to take. I would also do a water change right now. When in doubt, water change. This could be swim bladder disorder. It usually shows up as the fish floating on its side or upside down, but I had a goldfish with swim bladder treat presented symptoms like this. Does he appear to have any trouble swimming? Or is he just not doing it? If there’s no trouble swimming, I don’t really know what this is. The raised scales definitely could be a sign of dropsy. The person from this thread used KanaPlex, and it seems to have worked. Update if he gets better or worse.

 

I like to change 50% of the water every week to ensure no nitrate or ammonia buildup. You clearly don’t have this issue, but just for the future that might be a good precaution to take. I would also do a water change right now. When in doubt, water change. This could be swim bladder disorder. It usually shows up as the fish floating on its side or upside down, but I had a goldfish with swim bladder treat presented symptoms like this. Does he appear to have any trouble swimming? Or is he just not doing it? If there’s no trouble swimming, I don’t really know what this is. The raised scales definitely could be a sign of dropsy. The person from this thread used KanaPlex, and it seems to have worked. Update if he gets better or worse.

Thanks for the heads up. I think I might revert back to weekly water change.

It doesn't have any issues swimming. It just sinks to the bottom when it's not swimming.

And thanks for the KanaPlex link!

 

I know there is no “right” temp for goldfish but isnt 82 degrees to high for them? Maybe that’s the problem

 

I know there is no “right” temp for goldfish but isnt 82 degrees to high for them? Maybe that’s the problem

I'm aware of that but my other goldfish didn't exhibit any issues. I'll keep the temperature in mind.

I told the local fish shop owner about the ranchu. He said it doesn't look like dropsy and asked if the poop was clear. The poop wasn't a solid color, even when I fed peas, it's translucent green. So he recommended Prazi Gold medication (active ingredient Praziquentel). I hope it works.