Sick African Brown Knife

[thunderheartatx]

Request Help PRETTY PLEASE

Tank size: 80g
pH: 6.4
ammonia: 0
nitrite: 0
nitrate: 100
kH: 80
gH: 250
tank temp: 80 F

My first guess is nitrate poisoning. I admit that keeping fish has always been easy for me, I never lose them, so (until now) I don't test the parameters regularly, or change the water as often as some might suggest. My ABK stopped taking food a couple of days ago, but it didn't concern me until yesterday, when he became depressed, lost bouyancy (tilting forward), and constantly hovered over the air bubble pump as if he couldn't breathe. When he wasn't fighting to keep his equilibrium, he was resting on the bottom of the tank. Today (he lives at the office) I notice that the back half of one side of his body is speckled gray, where he is usually a rich, dark, blackish-brown all over, although that seems to be fading back to dark already. When I noticed these symptoms, I immediately pulled the test kit, and while all parameters were recorded as above, the nitrates were off the charts. Googling with one hand and changing water with the other, I did 5-10% water changes over 5 hours to reduce the nitrates and the tank temp, which was around 84 F (texas y'all). Will be doing slowly again today all day, he seems a little-but-not-much more stable, everyone else is fine.
 
What else can I do or not do to rescue him from the throes of death? I read that the bio cylinders in the filter may reduce nitrites but increase nitrates, so perhaps I should remove them? Is there another bio filter material I can replace them with? I had put in brand new carbon filters at the last water change. I bought methylene blue as a dip, as I read that it can speed recovery from poisoning and quickly increase blood oxygen level, but I'm worried that moving him will stress him further, although he is pretty comfortable with me. I'm also worried that if he is not suffering from toxicity that I could futher harm him by the process, since I don't know much about MB. Is it too late for him, should I prepare for the worst? Had him for seven years.
 
Thanks for reading and advice.
 

Volume and Frequency of water changes: 25% monthly

Chemical Additives or Media in your tank: carbon filters, bio cylinders

Tank inhabitants: ABK, senegal bichir, 3 silverdollars, African butterfly (everyone else as normal)

Recent additions to your tank (living or decoration): none

Exposure to chemicals: not that I'm aware

Digital photo (include if possible): He's been hiding since he got sick, but I've included the best photo I could get of his odd discoloration.

 

[N0body Of The Goat]

25% monthly water changes is rather low, I do ~50% weekly changes on my five active tanks, including my African Congo/oddball 6-footer.
 
Even though you have a nitrate reading of 100mg/l, it could be higher, the test reagents fall out of solution and give false lower results. Shaking/banging the bottle for a few minutes will help give truer readings.
 
Small daily (not hourly) water changes would be my approach at this point, with 10-20% changes each day for 2-3 weeks. Changing too much of the old water too quickly could risk "old tank syndrome" where the major water chemistry changes shock the fish fatally. You could empty the 10-20% out early on during the day and then gradually refill during the working day.
 
Might well be age related to a certain degree, having just googled lifespans. I always thought of African oddballs as all being long lived fish (eg. 30 years for Ctenopoma acutirostre), but when I googled lifespans after losing my African Butterfly last weekend, I found 5 years seems typical.
 
Not sure what else to suggest from the info given and the photo supplied. My Xenomystus nigri has been with me about two years now, ~14cm SL and has been very easy to feed (Tetra Prima; Hikari Cichid Gold floating pellets; Doromin; New Era Tropical Pellets etc.)