Black red-tailed shark stopped eating - any advice?

[robertiv]

I've had a 38g community freshwater tank for about a year and a half. One of my centerpiece fish is a black red-tailed shark who to my observation has completely stopped eating over the last couple months. It's possible he's eating at night when lights are out, but since beforehand he fed freely any time I'm doubting it's much.

For a short while last autumn he had some white spots on his gills, but they resolved without any action on my part and at the time his behavior was unaffected. He has always been a little shy but now is much moreso, not coming out of heavy cover unless it's directly disturbed. I have sent food that he previously loved right past him, no reaction.

I do a 50% water change weekly, all parameters are pretty well around normal levels. A couple months ago I realized my nitrates were high (80+ppm), but since then keep them around 20-40ppm. He has no physical signs of illness that I can see. I've tried both proven & new types of food, no luck.

I lost a few fish months ago (a couple danios & platys) and haven't restocked until I figure out the shark's issues, so it's not an overcrowding thing – he was perfectly happy with a more populated tank than right now.

Any advice? Anyone seen this behavior in other red-tailed sharks? I'm worried he's starving to death.

EDIT: have tried regular flake, sinking wafers & pellets both algae & regular, various other sinking foods, frozen brine shrimp, etc.

 

[Colin_T]

What is the ammonia, nitrite, nitrate and pH now?
What does the fish's poop look like?
Is the fish really fat?
Any chance of a picture of the fish and of the tank?

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You want nitrates as close to 0ppm as possible and under 20ppm at all time.

Wipe the inside of the glass down with a clean fish sponge.

Clean the filter if it hasn't been done in the last 2 weeks. Wash filter media/ materials in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn.

Do a 75% water change and gravel clean the substrate every day for a week.
Make sure any new water is free of chlorine/ chloramine before its added to the tank.