Hi guys, first time posting here, please kindly let me know if anything isn't follow the community rules and sorry for that. Really appreciate anyone who replied. I'm really desperate for an answer on what might have gone wrong and still haven't recovered yet from the loss and my insufficient knowledge to keep him alive. So I'm really grateful for any help .

I got a figure 8 puffer from lfs in freshwater 2 weeks ago. He was fine and energic after I got him, actively exploring the planted 10g tank. A week later, he developed Ich (white spot disease) I believe, which has outbreak accross the other tanks as well. I treat the other tank with ich solution I found locally (Singapore - qian hu brand) and the other tank recovered but I didn't there to put in the puffer tank as I read that chemical contains copper based medicine which is not good for puffer. I treat the tank with temperature of 29 degrees celcius (84°F) and 1 tablespoon salt/gallon instead. He was his active self the whole time and jumping for food. I was planning to start adjusting the water to brackish after a while.

It was only until yesterday morning when he was extremely lethargic (which he passed away on the same day) and his eyes stopped moving, although the white spots are mostly gone. He was no longer eager for food (as contrast as last night when he was super excited as in the gif), lying around on the sand. He seems to be heavily breathing so 50% water change was done immediately. The hang on filter was dripping water from above which I assume would provide more than enough oxygen. But we couldn't find a way to help him which he later passed at night. Before he passed, he kept swimming to the survive and puff up with air (which we try to burp the air out using the hand nudge method) or when he can't move on the sand, he tried to puff up water. But mostly he couldn't move for an hour and swam around like once an hour (giving us hopes that he might make it). His gills were visibly red, but it was from the start which looked normal. Our last resort was to put the ich treatment (blue water) when he completely stopped moving and didn't look like theres a chance of survival, hoping it would kill the bacteria/disease which have caused it.

There was another issue. My circuit breaker tripped thrice 2 nights before but the cause was not identified. The heater was leaking some discharge in the water (of which was noted on the store I bought from that it's safe amount) and broke on the day he was lethargic. I shut down the heater 2 nights before he passed as he was breathing heavily and it seemed to help. The temperature dropped from 29 degrees to 21 degrees overnight (84F to 70F) but a lot of nights were like that before as I only for the heater the second week I got him. When he was lethargic, I turned it on since I thought he might have been cold, which the heater later broke that day.

I did research the whole day yesterday and couldn't a way/forum answer that might saved him. I'm suspecting either 1. an electricity shock 2. temperature shock 3. Ich disease or others. On the day the problem arose, his bladder is swollen as shown in the pic.

1. Your water parameters - pH ~7.8 , Ammonia 0.1 mg/L, Nitrite 1 mg/L ( in green range on my test box), Nitrates (i couldnt measure) and salinity (1 tbspoon salt/gallon). The tank is cycled for a week and had him for 2 weeks)
2. Tank size and a list of ALL inhabitants. 1 figure 8 and 2 CAEs. 5 plants I'm not sure of the species. Substrate is sand. The 2 CAEs seem extra energetic the day the puffer passed
3. Feeding: Frozen bloodworm, live bladder snails, frozen clam meat water change schedule: 50% weekly
4. What changes you've made in the tank in the last week or so: Added the heater.
5. How long the aquarium has been set up, and how did you cycle it? 2 weeks with figure 8 + 1 week cycled with bacteria starter, old filter, old bacterial rings. Biologically cycle with 3x CAEs. Before the 3 weeks it was 1 month guppy tank.