Opinions on Aquarium Dividers?

[Fish Enthusiast]

I’ve heard a lot of mixed opinions on aquarium dividers especially for territorial species like bettas. Obviously I would never divide anything less than like a 20 gallon long. But I’m trying to decided if I should put a divider in a 40 gallon I have so I can house two separate bettas. However I’ve heard they can sense each other especially if the divider is not of good quality or see through. 
 

but just in general, I’ve heard circulation becomes poor for the side without oxygenation and heat also becomes an issue. Should I invest in dividers or do you think it causes more harm than good? 

[Pepere]

I would think it is easier just to get another small tank for one of them.

[Andy's Fish Den]

I have used a divider in the past in a 20 long for a couple of male bettas. the divider was black acrylic with small holes, but I had a small sponge filter on each side for filtration and flow. I never noticed where they would be flaring towards the other side, like they sensed the other one.

[Flumpweesel]

I've used them as a temp fix ( I used plastic canvas) no issues with flow this was for female bettas They could see each other but as they couldn't chase the bullied fish soon worked out it was safe.  

[Tanked]

I use ceiling light diffusers to separate plants from the larger plant eaters.  There is no restriction of water circulation, and small fish easily swim through the grate if they want to chill with the plants.  I don't keep bettas, so I don't know if seeing each other is an issue.

[Lennie]

Mixed opinions usually mean trying it is basically a high risk and you def should have a second plan if it doesn’t work and don’t exactly rely on it working solely based on the idea of success. Like if you wanna try, and see that it fails, what will you do? If you have an answer to how to deal with such situation, then you can give it a shot. If not, then I wouldn’t try.

Also if you ask me, I dont think it is all about seeing each other when it comes to fish’s senses. Sight is one element but not the only one. I even witnessed many times, when my fish get into breeding mood, even shrimp in the same tank get frisky and very active, the way males swimming around crazily searching for a female to breed.
I think there is a risk of them to feel each other even if they dont see each other. 
 

Some breeders keep bettas in different containers for a week or so, not even in the same tank, but just letting them see each other before getting them ready for spawning and being familiar with each other. I think it is very unlikely for bettas not to sense each other behind dividers sharing the same tank, but I never tried

 

Edited February 6 by Lennie