Yeah, unfortunately I experienced this as well.
I used to only keep non-planted, just decorated tanks with fish and all the fish species were very active and happy (no they were
not stressed, running from everything because of no plants to hide in/behind).
Once I got into planted tanks, I turned those non-planted tanks into moderately-heavy planted tanks and fish activity significantly slowed down. They became very reserved, not happily swimming about like they used to. Most would just end up hovering/staying place. I thought for sure the plants would make them even happier, seeing as how they have a new environment to explore, and with the plant cover, you would think the reduce lighting and sheer foliage would have them feel even safer to venture around even more and increase their playfulness throughout all the plants. Surprisingly to me, it was pretty much the opposite. I can't say they are stressed from the plants, but they definitely aren't acting very happy in comparison to how they were without plants. I thought maybe it was just the reduce flow/current from the plant obstructions and maybe they just needed time to settle into the new planted environment, but nope, still to this day they aren't nearly as active as they were in a non-planted tank. Even more evidence was when I did a massive trim, reducing plant load by 80-90% and fish activity spiked backed up, still not as much as before, but was a welcomed sight again. It's probably not the plants themselves, but just the space they would take up.
And just a FYI, I did have groups of 30 cardinals, 20 espei rasbora, 20 rummy nose tetra, and tons of Corydoras as far as schooling/shoaling fish, all of which were very active in non-planted tanks, but drastically became less active in a planted tank.
Just was a bummer to me. I really liked the beautiful scapes that I've always seen with planted tanks which is what drew me to it, but after experiencing the reduced fish activity with the plants, me being a fish guy over plants, it was a big turn off. I very much enjoyed the fish activity in a non-planted tank to a "less/inactive" planted tank (even with all the nice colors).
But since I paid hundreds for plants already, I decided to stick with the planted tanks for now.
Just my honest experience. It seems not everyone notices or experiences this same fish activity change, but there are some others who have also noticed this, so we aren't the only ones (so we aren't crazy)
