Syno Petricola Flashing

I have 2 syno's in my 29 gallon. Still small, they are going in a 55 eventually when it is setup. One swims around the caves I built out of slate and wonders the tank all the time the other just sits by the filter intake all day long. Scratches on plants and the tube just constantly. It's right against the glass so it's easy to get a good look. No ich no parasites no anything. Absolutely fine and nobody else in the tank flashes either. I haven't had new fish and this syno has been doing it for a really long time. Like I noticed it months ago and just told myself to keep a eye on her which I have and no signs of anything.

What could be causing this? Maybe because I only have 2 is the one bullying the other and stressing it out. I have hard water but not lake tang style super hard water. So maybe it's that? I really just don't understand what is wrong with this fish.

PH last time I checked was 7.8, GH was like 18.

Like you said it's the behaviour that worries me the most. There are 2 and at first they both hid by the filter intake. Then one got used to the tank and likes to swim from cave to cave and the other just hides. Every once in a while it will come out follow the other around and wonder the caves. But it's like 1 10th of the time is it actually acting normal. I plan on getting 2 more so I have 4 but don't have the tank space right now for it.