Drooping/Sinking Tail on male Emerald Dwarf Rasbora

[Wicklessfishmom]

I noticed this male Emerald Dwarf Rasbora swimming funny about two weeks ago. I tried to catch him to quarantine and figure out what was wrong with him, but he’s still a pretty efficient and fast swimmer. He’s still eating and doesn’t seem to be sick other than the way he swims differently than the others. He does stay more in the back than the healthier & stronger looking males. His coloring is still vibrant throughout the front 2/3rds of his body, his tail just looks paler and skinnier without any visible signs of injury or illness. Any thoughts on what might be wrong with him?

20 gallon established Heavily planted tank-

Stocked-emerald dwarf rasboras, ember tetras, rummynose tetras, cardinal tetras a few Otocinclus catfish and amano shrimp. 
water parameters-

temp-76°, ammonia-0, nitrite-0, nitrate-25, chlorine-0, GH-300, KH-120, PH-7.2

Edited September 1 by Wicklessfishmom

[Guppysnail]

I keep both emerald and celestial pearls. Occasionally they get injured sparring or mating. They get a bit rough with both.  Sometimes they just get old. The droopy tail and more jerky swimming is classic of these things. Sometimes they recover sometimes they do not. 
There is nothing to be done other than making certain he gets his share of food and does not get outcompeted.