Is this Denison barb ill?

[ACW]

I've had a group of Denison barbs for several months now. All of them have been healthy, but one has a very weird trait--its coloration keeps changing. When I got them, all had the same markings, but after 2 months, one began to lose its color entirely. After a few weeks, it looked like an albino. I tried and failed to catch it to medicate it, as it otherwise seemed completely healthy. It eats well, swims fast, is a similar size to the others I bought at the same time, etc. After a few days of having an albino, I noticed that its black stripe was returning, and within a couple of weeks, it looked mostly normal again.

Since then, it has regained and lost its color regularly, taking around a month to complete the cycle. The attached picture shows it halfway to recovering all color (with a non-color-morphing Denison at the top-left for comparison).

I'm guessing a stomach parasite maybe?

[Colu]

It could be internal parasites causing colour  changes in your fish  or it could be stress related  is it eating ok have you noticed it being bullied

[ACW]

No issues with eating--like all my Denisons, it eats fast and as much as it can get. Early on, I noticed it was getting bullied, but that seems to have stopped as all those barbs have gotten older. My nitrates run a little high due to bio-load (long story short, I have more fish in there than I planned on or paid for...), so I do regular water changes.

What really throws me though is that its color doesn't stabilize--it keeps cycling between 'fake albino' and standard.

[Colu]

I would treat his food with praziquantel powder and see if he improves

[ACW]

Thank you! I'll give it a try