Fin Biting and Lethargy/Not Eating?

[Neight]

Housing
I have a ten gallon that was divided to five last night.
It stands between eighty and eighty-two, depending on the house temperature. It is filtered, aerated and heated.
Timaeus previously lived with three platies, but when I noticed that he wasn't looking so good, I went out as soon as I could and made a divider.

Food
Timmy gets Omega One Floating Betta pellets and the Tetra food my platies eat, sometimes, too. I feed him twice to three times a day, one pellet each, though he hasn't been eating.
Maintenance
I do a fifty percent once a week. He's only been in this new tank one week, and today was the first change, as it's both larger and filtered than his previous housing unit, where the water was done fifty percent every day and fully once a week. {A one gallon fish bowl.} I use a water conditioner with PHE and a filter primer, since the tank was just set up.

Water Parameters:
The water hasn't yet been tested, as I need the money for a kit.

Symptoms and Treatment
I've noticed he has clamped fins, which he has also been chewing. He had some previous damage from being bullied at the pet store, and a little nip from one of his tank mates when they were being introduced, but it has worsened, and his colors have darkened.
He has also been quite lethargic, staying near the bottom of the tank or hiding in his pirate ship decoration. He still comes to see me when I approach the tank, and seems to have no trouble moving when he finds he needs air. This really began about three days ago, but I was naive enough to hope he would get better. He hasn't received any treatment yet, since I don't know what's plaguing him.
He's never been sick before, and was thriving.
I've had him for about a month, and he was fully grown when I bought him, so he's well into adult-hood.

I really, really need to know what's going on with my fish. In the short time I've had him, I've grown very attached to him, and I want to do everything I can for him. {I will try and post a picture later.}

[Oldfishlady]

Sounds like he stressed with the new tank and tank mates and this compromised his immune response.

What I would recommend-move him back to his 1gal unfiltered tank and start a fresh/dechlorinated water treatment.
Make 50% daily water changes for 2 days-then 50% every other day for 4 days and see if this perks him up and gets him eating. If not eating by day 3 or he has symptom or behavior changes. He might need a salt and tannin treatment.

If you have a tannin source-either IAL or dried Oak leaf-I would add this to the 1gal and have some premixed/steeping in a jug of dechlorinated water to use for water changes.

Nutrition-good quality varied diet fed in small frequent meals and if you have access to live mosquito larva-offer several rinsed a couple of times a day to boost protein intake to support the immune response.

Keep us posted....

[Neight]

Thank you so much! I was considering moving him back to the smaller space, but I wasn't sure. I'll keep posting on his progress.