Ropefish very sick, diagnosis and treatment advice?

[Oscicherum]

So I was away from my tank for about two weeks, and had one of my good friends feed my two bichirs and ropefish, with a bristlenose pleco. When I get back, the ropefish is lethargic, not eating and covered in a weird white fuzz. I tested the water and SURPRISE, the nitrates were through the roof due to my pal overfeeding them! To be fair I don't blame him because he is rather new and hasn't kept anything other than a few angelfish, but that's beside the point.

Here's the main problem: I have NO idea what this could be. The other bichirs are healthy as can be (if a little plump) but the ropefish is not eating. He's never been the kind of fish to eat with the lights on, if at all, but usually I hand feed him. The pictures below (if I can get them to work) show some pretty nasty stuff on the aquarium floor, as I just got back a few hours ago and haven't done a good water change (another mistake of going away for two weeks). I will do one right after I post this though.

Now I have no idea what it is, so I have no clue how to treat it. Polypterus have always been super hardy and resistant so this is new to me. If anyone can help it would be truly appreciated! I've heard salt and temperature is okay with polypterus, but keep in mind there is a bristlenose...

Here are some numbers:

Ammonia=0
Nitrite=0
Nitrate=80 (yikes!)

Running a Fluval 406. Loads of beneficial bacteria.

The temperature is currently at 80 degrees Fahrenheit and I have two juvenile bichirs and one ropefish, plus a bristlenose pleco for cleanup of the algae. This is the first time the tank has gone without a water change for more than a week and I'm about to do a few 50% changes over the next couple of days. No idea how I will get the nitrates back down to 5-10 ppm but I'll try my darn best.

 

[kno4te]

Likely just because of a nitrate poisoning. The row fish probably needs a lot of jus likely just because of a nitrate poisoning. The real fish probably needs super clean water and salt at 1 teaspoon per gallon. Hope it pulls through. Would add maybe just some API fungus cure as well just to cover any kind of potential fungus is what it sounds like. Post some pics when you get a chance.

 

[kno4te]

Would move to a qt tank if able to control the water parameters and med dosing a little easier. If ur able to.

 

[Audaxcity]

I've seen the same fuzz on the live gobies at my supermarket.
Gonna guess its nitrate poisoning as well.
If you think you can, catch the fish and pick some of the fuzz off. Don't do it if you think you might remove some of the slime coat though.

Dose the entire tank with epsom salt. Its anti-fungal. About 1tbsp/10 gallons, but you need to find the optimum range yourself. Its magical stuff, solved most problems I came across in my hobby.