Strange fish behavior

Hey guys,

hoping someone can help me out with some erratic fish behavior. I have a 15 gallon fluval flex with stock lighting and a modded filter pump to reduce flow down to 5-6x turnover per hour. Tank has been up for 5 weeks and water parameters are 0ammonia 0nitrite and 20nitrate. My water is very hard at approx 500TDS with alkalinity and hardness out of testable range on my strips. PH 8.0-8.4. Tank is planted in seachem black fluorite sand with pressurized CO2 at 1BPS.

I have had a betta in since start up and added 2 Nerite snails 2 weeks in, a clown pleco 4 weeks in and finally 8 harlequin rasboras 2 days ago. No casualties and cycled quickly with media from an old tank.

now here’s the problem: my betta has been glad surfing pretty hard and glaring at the glass, but I thought that was just his personality. Since putting in my rasboras last night they haven’t left the glass either. Looks like I’m having trouble with glare/reflections but I’ve tried covering the side with a blanket, moving the tank, adjusting ambient lighting to no avail. Pretty annoying as I never see the fish swim around this nice planted home I’ve made for them...

could this be due to stress with the hard water and high pH? Should I cut with RO? Have kept many tanks before and never experienced this problem so I’m thinking it the glass, but I’ve seen many people on YouTube with the same tank and can’t find much history on the issue. Picked up the flex as I thought it would be a nice nano but becoming disappointed...

Just a couple of guesses as that's all we can do (cuz the pesky fish won't talk to us)....

I'm guessing it's stress. Harlequin Rasboras prefer near neutral water so any swing - either acidic or alkaline is not ideal. Your pH is pretty high for them and for the betta. I do not have much experience with the effects of hard water but assuming this could contribute to their behavior as well.

The betta may also be causing some stress to the rasboras and vice versa. If your betta is flaring at his reaction in the glass he might be one of those aggressive types that prefers to have the tank to himself.

The rasboras may need more of a long vertical tank to swim vs. the horizontal orientation of the flex. They may be feeling a little cramped.

I have had luck bringing my high pH down with Fluval peat granules. I add it to my 5 gallon container that I use for water changes. It's brought mine down from 8.3 out of the tap to about 7.6 - 7.8. That might be an option to help with your pH. I'm not sure if anything can be done about the hardness. And ultimately your tankmates might not be able to coexist. Maybe add more plants to break up line of sight and give more places to hide?

Just a few thoughts. Hope everything works out. I had a lot of glass surfing from new fish in the past and they settled after a day or so. Hopefully others will chime in with advice. Keep us posted on how things are going!