Stressed Otocinclus...why?

I keep a 2.5 gallon cycled freshwater tank with a light bioload (2 ghost shrimp, a snail, an otocinclus, and 3 tetras) that has been doing very well the past 6 months. I do around a 40% water change a week, feed daily with hikari algae wafers and tropical pellets. It's filtered, heated, and well-maintained.

Recently, my oto started to lose color. He is still very active, and darts around when tankmates get too close, and acts normal in every way, but his color is definitely not normal. I've checked all my parameters; ph is fine, nitrate level is very low, food sources are available, light levels are stable... I don't know what is going on. He has lost all of his color, the stripe is now gone, and he is completely white and translucent, but acts normal. During the day, he often hides in the cave decoration in the middle of the tank.

I can't think of a reason why this is happening. I *did* make a change to the filter a few months ago where I took the replaceable carbon sponge out. I had been mildly dosing excel when I experimented with a moss wall background. I never replaced the carbon, and I kept reusing the white batting -- washing it out every two weeks. I know washing it out may disrupt the bacteria living in it, but since I keep my water/substrate undisturbed, my nitrite levels haven't been messed up.

Any ideas? Should I put in new batting/carbon sponge to see if that makes a difference? Logically, since my water parameters have not changed, I'm not sure why this should make a difference, but it is the only thing I can think of that has changed.