My betta had this spot before I moved him into the 20 gallon tank. Previously he was in a 10 gallon tank with 4 black skirt tetras. He has done perfectly fine with the black skirts for a year with no fin nipping or aggressiveness on either side. I started with him and 3 black skirts. He only flared at himself in a reflection when I first got him and put him in the tank (the 10 gallon tank) and once when I added the fourth he flared once at it as soon as I added it. After that there have never been any problems.
The dwarf gourami, white clouds, pristellas, ghost shrimp, and 2 black skirts are all new.
When I did the 20 gallon tank upgrade I cycled the tank for 24 hours with no fish in it. Then I added Microbe-lift special blend, which is an ecosystem bacteria treatment for both salt and fresh water aquariums to help cycle them and prepare them for fish.
Then I added 4 of the new pristellas, 2 of the new black skirts, and the 2 new ghost shrimp. Then I waited 24 hours and added 2 more new pristellas, 1 more new blackskirt, a corey catfish (who didn't make it), and the dwarf gourami.
Then I waited 2 more days before I transferred my betta and 3 black skirts he had been living with for a year over to the 20 gallon tank.
After treating the 10 gallon tank repeatedly and this spot on my betta not going away (although my tetras were fine after one treatment) and doing the water changes I was concerned that there may be something in the water itself that just wouldn't die off. So I decided rather than doing a 100% water change that I would upgrade to a larger tank and prep it for the move to alleviate stress on my betta and tetras and reduce chances of them getting sick or dying from the tank change.
I have been testing all levels (PH, amonia, nitrites, nitrates, etc) in my tank since I first started cycling it. I tested the 10 gallon tank as well so I could compare the 2 tanks and waited until the PH levels were the same to transfer them. I have tested the new 20 gallon tank a total of 4 times in the last 11 days (which is how long I have had the 20 gallon setup) and all of my levels are good. My temperature also remains between 78 and 80.
My betta actually became more active and seems happier since I put him in the 20 gallon tank. But even after treating the 20 gallon tank the spot still didn't go away, which is when and why I posted here asking for help.
As you can see in the pictures I have plenty of hiding spots and room for them all to live together. I researched quite a bit and spoke to the aquarium store (I got my new fish at an aquarium store, not a chain pet store) before deciding to get the dwarf gourami.
I prefer not to go by the standard scientific view when it comes to mixing fish. I prefer to go with others' experience. We are dealing with living creatures after all and they each have their own personalities. Just like people, some bettas will have bad tempers and fight easily, others are more docile. I happen to have a more docile betta. I know his personality. I did not make my choice to buy the black skirts or the gourami on a whim, nor did I make it lightly. I discussed it with the stores before I purchased them and both stores gave me the option to return the fish if a problem arose with the betta. No problems ever arose. Now that's not to say that I would put angel fish or fancy guppies in because I have never seen mixed reviews on fish like that. There are mixed reviews on black skirts and dwarf gouramis, though.
I have read the mixed reviews about mixing dwarf gouramis with bettas. Some people mix up gouramis with dwarf gouramis. Dwarf gouramis are less aggressive than normal gouramis and tend to be more shy. They usually do well with other dwarf gouramis in their tank but can get territorial with other males dwarf gouramis if they don't have enough room or hiding spots.
There are mixed opinions and experiences with putting dwarf gouramis in with bettas. Some people have had problems and others have had no problems at all. I have had no problems with my gourami and betta at all. In fact, they like to hang out together sometimes. They will both chill with each other peacefully in the plants. Other times, they part ways and each has claimed a cave for himself. My betta uses the sphynx cave and my gourami uses the pyramid cave. I also have plenty of plants for both of them to utilize and they do. My betta never even flared at the gourami and the gourami has never gone after my betta. This is my personal experience and I see no reason to remove my betta or gourami if they are living in harmony together. If a problem does arise then I have another tank ready to house one of them.
As for the black skirts, I feel the same as I explained with my gourami. I've had great success with my black skirts and betta together. If you research you will find that black skirts typically don't fin nip or act aggressively if you keep them in a large enough school (of at least 6). In my 10 gallon tank I only kept a school of 4 but they kept to themselves and I never had problems with them. In my 20 gallon tank I now have 6 and they also school with the pristella tetras. They completely ignore my betta and my gourami. They tend to hang out in the middle of the tank both horizontally and vertically. My betta likes to hang out on the bottom and always has. He comes out and swims up to get air, to eat, and when he sees me. Again, I see no need to remove my fish if they are living together in harmony.
On another note, I'll add that every website I came across that listed companions for bettas I saw white clouds listed. Yet the girl at the store told me she wasn't supposed to sell them to me if I had a betta and insisted that bettas would kill them. But she did sell them to me because I frequent the store and she wanted feedback on how they did together. I've also found some sites say that gouramis will kill them. Well, my white clouds stay in the middle with the tetras or at the top of the aquarium level, school together, and I've had zero problems with them and my betta or my gourami.
I believe compatibility depends much more on the fish's individual personality, the tank size, and how the tank is set up with plants, decorations, etc. This is something I have read, was told by an aquarium expert, and have experienced myself personally so I'm sticking with it. If I run into a problem, I am prepared with a plan B to separate them.
Here is my aquarium setup:
Other side view
Betta and Dwarf Gourami together
Betta and Dwarf Gourami swimming together (he is not flaring here)
Betta and Dwarf Gourami together
My betta in one of my live plants
My betta in one of my fake plants