Timid Silvertip Tetras

[aquatoid]

I have a problem (?) with my school of silvertip tetras. They have become very timid. Whenever I move near the aquarium, or stand up from the chair that is 3 meters (10ish feet) from the tank they seem to get scared and huddle in the area right of the biggest rock in the image.

Sorry about the potato photo, I had to take it at maximum "zoom" to capture their currently natural behavior when everything is fine for them. They point towards the Eheim Powerline XL which is to the right. Basically they are chilling in the stream from the spray bar. The flow isn't insanely strong, as I have customized the spay bar making it longer and with larger holes. The color of water is due to me using Blackwater Aquatan in the tank.

Some background info: 

The tank is a 525 liter one with a relatively large bottom area. I have Fluval FX-6 and Eheim Powerline XL both with a pimped spraybars moving the water. Pimping is done mainly to tame the flow a bit more so the sand doesn't get blown all over the place in the tank. Temps are 24,3 C near the front glass to 25,5 C at the back near the heater. pH is 7,13 at the moment, but the target for me is 6,9. I have been increasing the hardness a bit lately (might explain something), which has pushed the pH up a bit. If my JBL test strips are to be trusted at all, KH is now just about 5 or so, and GH still does not register on the strip. Or rather, only the edges of the first pad are slightly colored. This isn't a shocking thing, the stuff I'm currently using is Sera KH plus. I have ordered  Sera KH/GH plus to be able to affect both values in the future. NO3 = 25 mg/l and NO2 and Cl are both 0.

I started with 20 silvertips, and they turned out to be fairly territorial little monsters towards each other. Might have been just something to do with sorting out their pecking order in a new tank, but they seemed to stake out claims in certain areas of the tank, only coming together when feeding or getting stressed by a water change. 

I decided to get some more to get them to perhaps school a bit more, and added another 20 into the tank. The aggression seemed to die down a bit, at least at first, but it seems there are one or two silvertips that really like to watch the world burn, and fought anyone that got close to them. 

Then something changed, they started to become more withdrawing and jumpy. Before they used to come towards the surface and the front of the glass when I got near the aquarium, basically they had figured out I'm the source of food. Back then they used to mill around in the middle of the aquarium and came to me. Now they hang around the front of the glass and shy away. They even (at least some of them) seem too scared to eat. They used to be tiny piranhas when I fed them, now I'm not even sure all of them eat at all. 

Only things I have done is increase the KH a bit, over the week or so. I also directed the spraybar of the Eheim Powerline a bit more towards the front as I increased the size of the holes on the bar. So they seemed to go with the flow, but that shouldn't be the main criteria, or even couldn't be, as the flow is stronger at the top, and none of the silvertips is drawn to the flow of the FX-6. 

Could this be an oxygen thing? I know raising KH buffers the water for pH swings, and it seems more CO2 can be dissolved into it. I'm feeding the tank some CO2 (monitored by JBL pH computer), and it seems the diffuser has been running more with higher hardness. There is a lot of surface agitation going on, so the oxygen shouldn't be an issue.

Any ideas would be much appreciated. I have patience to wait and see how this will develop, but I don't want my fish to be distressed or suffering while I do so.

[Streetwise]

Is it possible that you made a change in the room lighting, and you might now be casting a shadow into the tank when you approach?

[aquatoid]

31 minutes ago, Streetwise said:

Is it possible that you made a change in the room lighting, and you might now be casting a shadow into the tank when you approach?

That thing hasn't changed, one of the things I can be certain of. The aquarium is the brightest light in the room in fact. 

Edited January 30, 2021 by aquatoid