Haha thought I was the only one!
I've literally just mentioned this shortly after turning my non-planted 100 gallon into heavily planted.
Read my posts here (#11 and 15) https://www.plantedtank.net/forums/8-...-cohesion.html
I've always been under the impression that the fish would be MORE active since they feel safer with all the cover to hide in, so they would be more adept to play around and do other activities, and have virtually no stress (no need to really stay on guard, keeping a eye out) at all since a safe retreat is right near them and they aren't as exposed.
Especially with the added benefit of the plants acting as biofilters and supplying more oxygen, you would think that the fish would be even more happier and active, but seems rather opposite.
I got into this hobby because of my interest in the fish, so I am more of a fish guy and have always had non-planted tanks filled with activity. But I recently decided to try out high tech (co2 not even installed yet, still shipping, so co2 effects aren't a problem) heavily planted tanks because they do look really nice, but I was really bummed when I see the activity levels of my fish lowered significantly. Not saying they don't move at all, just that the amount of activity in the tank has died down a lot. It seems like I've made them unhappy/depressed now, and me being a fish guy over plants, really does bum me out and kind of regret planting the tank. If I could go back, I would have stayed non-planted, but I think I will stick with the high tech for a little while since I did just order everything (really didn't expect activity levels to drop).
I've kept non-planted tanks for a long time, and just new to planted tanks, so maybe I am more noticeable to the behavior/activity differences (which are significant) with the two different tank set ups, which might be why others don't notice the lower activity levels. Again, all my parameters check out fine, ruled out many things, gave time for fish to adjust to new environment, even plenty of open area, even not too dense planting so they can move comfortably through the foliage, etc...just seems the presence of the plants is causing the difference in behavior.
As I mentioned in the other posts on another thread, I do have cardinal tetras who would just hover because they feel safe/comfortable, but my espei raboras and rummynose tetras are natural tight schoolers and would happily school in my tanks nearly all day and not because they were schooling in fear. I do observe/closely examine the fishes' behavior (many tell tale signs of how a fish is feeling, just got to know what to look for and what behavior means what) and I am positive my fish were happier and more active in my non-planted tanks (granted I did have a few plants in some tanks, but very minimal). I can tell the difference if they are schooling in fear or in happiness. Even my Corys are less active (not the most active fish, but I still notice a decrease in activity). ALL my species of fish activity levels have plummeted (my Corys are still spawning though, which has always been happening) and I haven't noticed ANY fish that increased activity levels. It's not like the fish ARE active, but I just don't see them, because I can/do see them and they aren't active.
Not sure what to equate it to. I've had non-planted tanks with a ton of decorations and the fish were still plenty active, so I can't really the decreased activity to the plants taking up space. I've ruled out co2 and nutrients as well. Everything points to plants having positive effects and more active behavior, but it apparently is working out that way. Guess it might not be a "parameter" issue and it's just the plants themselves (not like the fish are scared of the plants or anything though). I do know some of my fish are from natural areas with no plant life, but still, even the ones that are from planted areas are having decreased activity (not active like how fish behave in their natural habitats). I have had tanks with just a few plants with still very active fish, but I guess it's like the OP said, it seems the more plants, the less active fish are (even with plenty of open area).
Sorry for the long post haha
Just my experience. Good to know I'm not the only one noticing this difference, as I was beginning to think others were thinking of me as some kind of insane person with my observations hahaha