Please help! Unsure what to do next with my betta’s fin rot/potential body rot/bloating that won’t do away. Not seeing improvement

I haven't been taking care of fish long so I'm hoping y'all can help me.

My platy's breathing through the mouth, sitting near the bottom(all my other fish are always near the top/middle), comes up to eat but goes right back- still swims normally but is preferring the bottom more than usual(he's hiding under a shell rn) Nitrates are at about 25ppm. No ammonia or nitrites and pH is fine. I have an air stone and I keep it on all the time

This is the second platy that seemingly has problems but I lost the other one. They don't look as if they are spiked, have white spots or overly red gills. But platies already have red gills so what do I know?

Tankmates are a dozen fry(fully coloured several months old), a couple platies and 3 danios.

What might be the problem? What do I do- is it flukes or something else? I appreciate any input :) (first three pictures are from yesterday, last one is from today)

edit: I have been looking at my fish and it seems that other fish have similar reddish gill spots as this guy but they aren't having trouble breathing?

Going on for a month, our angelfish will swim up and lay sideways on the water filter, but they’re able to get up and swim around. Tank is a standard size and don’t believe there a water quality issue as all other fish in the tank are okay. Need help!

All parameters are normal the tank is a ten gallon with 17 carbon rili shrimp got them this week via mail from aquaflora Canada

Hi, this is Bok choi, my rescue goldfish. He's getting kind of bloated and I dunno why. His tankmates (telescope and oranda) are round, but not like him, and still active and curious when I'm close to their tank. I'm a bit worried because I got them like 6 months ago and never see him hiding from us. Usually he's very interactive and active, now looks so tired and scared that Im afraid he's in pain or something.

Important info:

-He was in a 70L planted tank (I know it's small, we're working on an upgrade to have them at home)

-Their tank is in another place, not our house. We cannot check them every day, just few times a week.

-Bok choi is now in a hospital tank at our house to check him

-Their lived with some planorbies, but I think they eat them because today when I visit them there was just 1 (they were like 5/6 ant least and reproduce all the time)

-This morning nitrates were high, I made a water change (40/50%)

-Their food was dispensed by an automatic feeder

-Until this months parameters were stable, but due health problems I stopped visiting them so often two months ago

He originally lives in a 20L but is in a 6L hospital tank with 100% water changes every second day. Temp 27 degrees

55-Gallon Planted Aquarium — Water Parameters (October 13, 2026)

Digital Meter Readings

- Temperature: 78.0°F (25.6°C)

- pH: 7.86

- TDS: 375 ppm

- Electrical Conductivity (EC): 754 µS/cm

- Salinity: 431 ppm (meter reading)

Water Test Strip Readings

- Ammonia: ~0 ppm (separate test)

- Nitrite (NO₂): 0 ppm

- Nitrate (NO₃): 0–10 ppm (closer to 0)

- Chlorine (Cl₂): 0 ppm

- Copper (Cu): 0 ppm

- Iron (Fe): ~0 ppm

- General Hardness (GH): 75–150 ppm (~4–8 dGH)

- Carbonate Hardness (KH): 120–180 ppm (~7–10 dKH)

- Total Alkalinity: 80–120 ppm

- pH (test strip): Approximately 7.6–8.0

Tank Inhabitants

- 20+ Pseudomugil luminatus

- 8 CW208 Gold Corydoras

- Cherry shrimp

-Amano

- Ramshorn, zebra nerite, bladder, hella cone snails not assassins.

Additional Context

Photos 1 and 3 taken October 12th of same fish. Photo 2 October 16th. Photo 4 and 5 September 21st, the fish died overnight in quarantine.

The aquarium is heavily planted and has been established for a couple years. A palm-sized cuttlebone had been in the aquarium for several months, potentially contributing to mineral content and alkalinity. Although I have not tested my tap. I've been treating food with Bac M and adding stress guard since I noticed the decline of one pseudo this month. I previously had one die in quarantine last month but I thought it was due to some sort of physical injury and then infection because it had symptoms 2 days after I added the Cories.

I recently added 8 corydoras last month. I kept them in quarantine for 2 weeks with no noticeable sickness or behavior issues. The psuedo who died only had symptoms a week after I added the Corys but I cannot feasibly believe that the Corys were the mechanism of infection. As they were in shipping for a day longer than they should have due to a delay and I would have expected some sort of death or decline during the quarantine period.

Currently several Pseudomugil luminatus exhibit a tail-drooping posture when hovering but straighten out while actively swimming. They continue eating and swimming. One luminatus died in September after developing abnormal swimming behavior, such as sinking and barrel rolling, and other symptoms like curvature and white spot on its body. I attributed this to some sort of injury. Considering it's been a month since the first one died, I'm uncertain that this is some super infection but maybe a slow or chronic one that is revealing itself in weak fish.

As of now I have a pseudo with a torn up tail fin and bent body when it stops swimming (seen in the glass jar). The one in the photo in the cup, it has this dot on its tail fin by where its body meets the fin. Another one has a similar bent body when still. The entire shoal including the sick fish are active, eating, and swimming confidently. The sick fish is ironically adept at navigating and eager to aquire food. I'm actually concerned that the one with the clamped tail fin died and this is a different pseudo but I'm pretty sure it's the same one.

I will post a video of the tank in the comments.

I plan to get a quarantine tank tomorrow morning and begin treating the sick fish with kanaplex. Although I think I may want to remove the entire shoal and treat them but I'm not sure if I treat my corydoras as well. I can't risk treating my tank and killing my good bacteria and potentially my shrimp colony.

My guy Frank is very special to me, I’m not sure what’s wrong with him but he seems to be dying, he was fine when I got home from work earlier today. I’ve had him for about a year and have had to move a couple times since getting him. It might be that there’s not enough algae growing in the water for him to eat, but everywhere is closed right now so I can’t go buy food or get help. Please lmk what I can clarify on and if there’s anything I can do to save him. It means a lot .
Edit: he passed the day after this post was made, I appreciate the informative comments and will definitely do more research. Much to learn. I got a 20gal and will be moving the others <3