Welcome to MFK!
Sorry for your losses. I think your nitrate is extremely high and you aren't changing enough water weekly. I would suggest slowly upping the percentage of water you are changing weekly. Start with 25% ever 3rd day until your nitrate lowers and then you should be good at 40% to 50% weekly, depending on the stocking level of your tanks.
The only bubble I see on the fish is at the vent/anus area, is that correct? If so that is usually due to either water quality or constipation.
Can you provide more details on what you are feeding, if you are adding any buffers to the aquarium to adjust your tap water parameters, etc.?
Have you tested your tap water for nitrate, pH and carbonate hardness (KH) by letting it off gas for 24 hours?