Sick platy, please help!

[Snailfish4]

Hey everyone, I’m a new fish keeper and one of my platys has fallen sick. Any help would be really appreciated!
Tank
What is the water volume of the tank? 20 gal
How long has the tank been running? 2.5 months
Does it have a filter? Yes
Does it have a heater? No
What is the water temperature? ~78-80
What is the entire stocking of this tank? 2 platys

Maintenance
How often do you change the water? Weekly
How much of the water do you change? 25%
What do you use to treat your water? Prime
Do you vacuum the substrate or just the water? Vacuum substrate

*Parameters - Very Important
Did you cycle your tank before adding fish? No, fish in cycle
What do you use to test the water? API freshwater liquid test kit
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.
Ammonia: 0
Nitrite: 0
Nitrate: 20
pH: 8

Feeding
How often do you feed your fish? Once daily
How much do you feed your fish? Very small pinch or flake food, or a few pellets each
What brand of food do you feed your fish? Hikari (used aqueon a couple times, because I had a sample and no other food, I know it’s not ideal)
Do you feed frozen? No
Do you feed freeze-dried foods? No

Illness & Symptoms
How long have you had this fish? 2.5 months
How long ago did you first notice these symptoms? A day ago
In a few words, can you explain the symptoms? Floats mostly at top, sometimes bottom; White gills (but his gills have always been kinda white); not moving a lot; occasionally swim vertical or slightly tipped sideways; saw him rubbing against a plant once
Have you started any treatment for the illness? No
Was your fish physically ill or injured upon purchase? No
How has its behavior and appearance changed, if at all? He used to bully the other (male) platy, now he is being bullied! He doesn’t eat, even when food is easily accessible to him, noticeably skinnier. Not swimming around much.

Explain your emergency situation in detail.
(Please give a clear explanation of what is going on, include details from the beginning of the illness leading up to now)
Yesterday and today, symptoms constant. I recently completed a fish in cycle with him but he seemed fine then. I move around a couple of decorations, his tank mate (male platy) started bullying him. (Btw the other one is fine). He started not eating at all last night, ignoring the food, just floating near the tank bottom. Saw him do a full 360 turn on his side. Gills look slightly swollen and more white than usual. Clamped fins as well. Been laying in/on a plant for the last 30 seconds.
Thank you so much for any replies, I’m new to this!

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[Bruce Leyland-Jones]

A very detailed post and sorry your fish is in distress...and sorry no-one has responded before me.

Your water chemistry seems okay and your pH and temp., whilst at the upper end of Platy preferability, don't appear to be problematic.
(Usually, a water chemistry/parameter issue would affect all fish, unless one has been particularly stressed.

From your description, it is possible that stress is a causative factor and I note your photo shows the fish 'hiding' in a plant.
(I'm a strong believer in a well-planted tank and one of the reasons is the security that living vegetation seems to provide).

I'm also a strong believer in the use of aquarium salt. Please check this video on Aquarium Salt from KGTropicals, to save me typing.

The pale gills suggests a lack of oxygen reaching the fish and excess mucus (stress) on the gills can cause this, as well as a number of illnesses/diseases.
You'll find that few members in this Forum advocate medications unless it's absolutely necessary...we're talking chemicals, after all.
In this case, it might be worth increasing the temperature a couple of degrees for a couple of days and adding a few more plants.

Keep us informed and good luck.

 

[Snailfish4]

A very detailed post and sorry your fish is in distress...and sorry no-one has responded before me.

Your water chemistry seems okay and your pH and temp., whilst at the upper end of Platy preferability, don't appear to be problematic.
(Usually, a water chemistry/parameter issue would affect all fish, unless one has been particularly stressed.

From your description, it is possible that stress is a causative factor and I note your photo shows the fish 'hiding' in a plant.
(I'm a strong believer in a well-planted tank and one of the reasons is the security that living vegetation seems to provide).

I'm also a strong believer in the use of aquarium salt. Please check this video on Aquarium Salt from KGTropicals, to save me typing.

The pale gills suggests a lack of oxygen reaching the fish and excess mucus (stress) on the gills can cause this, as well as a number of illnesses/diseases.
You'll find that few members in this Forum advocate medications unless it's absolutely necessary...we're talking chemicals, after all.
In this case, it might be worth increasing the temperature a couple of degrees for a couple of days and adding a few more plants.

Keep us informed and good luck.

Thank you so much for your help! Unfortunately my fish has already passed away, right as I was beginning treatment