Help With Sick Oscar!

I posted this before about two months ago but no one gave me any advice. I tried an anti-protazoan since the issue seems to be only with his scales, and it seemed like it might have helped the problem somewhat but not much.

Tank

What is the water volume of the tank? 75 gallon (upgrading to a 150 gallon in July when I move)
How long has the tank been running? A bit over a year
Does it have a filter? Yes
Does it have a heater? Yes
What is the water temperature? 78F
What is the entire stocking of this tank? (Please list all fish and inverts.)
1 Oscar
1 half-banded eel
3 large mystery snails

Maintenance
How often do you change the water? Weekly
How much of the water do you change? 50-75%
What do you use to treat your water? Prime
Do you vacuum the substrate or just the water? Both

*Parameters - Very Important
What do you use to test the water? API Test Kit
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.

Ammonia: .25 (just did a water change last night and Prime always shows up for about 24-48 hours as .25 ammonia. My tank has been fully cycled for a year
Nitrite: 0
Nitrate: <10 (never goes above around 20...ever)
pH: 7.2

Feeding
How often do you feed your fish? Once per day
What brand of food do you feed your fish? HikarI or Omega One for frozen
Do you feed frozen or freeze-dried foods? I feed about 80% frozen food, about 10% pellets, and 10% crickets or worms, just depends on the day

Illness & Symptoms
How long have you had this fish? 1 year
How long ago did you first notice these symptoms? December, but seemed to go away in Jan and Feb
In a few words, can you explain the symptoms? Started with a weird white sheen on his head, almost like the skin was shedding or something. He was also flashing a bit more than just the occasional itch. For a month or two it faded so I ignored the issue and just continued to monitor and kept a close eye on all water parameters (including phosphate, gh, and kh). Now the white sheen is completely covering his head and he has been rubbing his scales off.
Have you started any treatment for the illness? Tried anti-protozoan, didn't really help
Was your fish physically ill or injured upon purchase? No
How has its behavior and appearance changed, if at all? There is a white sheen on his head and missing scales. He seems to get very aggressive toward objects near his tank (People, phones, cat) when the symptoms are really bad.

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)
Back in December I noticed a white sheen (pics below-sorry if they aren't clear, as I mentioned he has been extra aggressive and kept trying to attack the glass when I put my phone up) on my oscar's head with some flashing. I asked on the forum in January but no one knew what was going on. I used an anti-protozoan to treat the tank along with not feeding for a week and doing 25% water changes every other day for about 2 weeks. It seemed like there was a bit of improvement (no more flashing and the spots seemed to fade a bit but it didn't seem to be a protozoan). As of the last week or so the white sheen is covering his entire head, he has been flashing quite a bit, and most disturbingly he has been rubbing his scales off and banging himself up

I did 2 extra 50% WCs this week but all of the parameters are fine, no livestock has been added to the tank since..September (1 mystery snail) and I added floating plants after the anti-protozoan treatment but not before the issues started. Neither the snails not the eel have had any symptoms at all. None of the fish in my other tanks have any symptoms, including the tank with a pleco and giant danios that shares a bucket with this one (I know I shouldn't share buckets, but I don't have closet space for more than 3).

This tank does have some small brown planaria, I see a couple sometimes after I do a WC/gravel vac. They seem to be about .5cm long or smaller. I could treat for them if anyone thinks they are in someway related but I have snails in the tank and don't want to harm them.

Sorry for the million details, but I wanted to give everyone a clear picture

Edit: I should add that there is NO carbon in the filter as I know that is sometimes listed as having the potential to cause HLLE/Hole in the Head.

 

That shedding skin appearence made me think velvet, one of symptoms might be "shedding". Check images if it fits?

 

But the sheen, and it is a sheen and not tiny dots, is white and not gold colored. Is there another type of velvet that is entirely white and not yellow/gold/orange/rust? I'm not familiar with it if there is.
Also when I was treating with anti-protozoans I believe the medication I used was for both Ich and velvet, so I assume that would have taken care of it if that was the case.

Edit: Since this has been going on since at least December, I think if it was velvet there would be additional symptoms of the disease and my other fish would probably have gotten it, right?
But if there is a different type of velvet please let me know.

 

Not different type, but if it gets bad, it might look like shedding layer of skin.

Velvet Disease - Fish disease

 

Not different type, but if it gets bad, it might look like shedding layer of skin.

Velvet Disease - Fish disease

Does velvet not always present first with the yellow/gold color? He has had the white sheen since December and just recently began rubbing his scales off. But prior to December he didn't have any gold or yellow sheen. He hasn't had any lethargy, faster beathing, or lack of appetite normally associated with the advances stages of the disease either.