The filter is what eliminates the ammonia and nitrites and other wastes so how would it be bad for bettas? I think what you are referring to is the tendency for naughty bettas to love to get into tight spaces and get stuck.
Sometimes if the filter is too close to the back wall I have known of bettas who have gotten stuck or had fins torn if the flow is too strong because those fins are just like big sails and can be caught in too strong an intake flow and torn but they will heal with clean, very clean conditions and proper care. Stuck bettas are a different problem. You need to make sure that any tight places and holes in decorations meet the "RULE OF THUMB".
The Rule of Thumb is very simple. If your thumb goes in and out of some place or hole in a decoration the betta will fit too. If not then it is too tight for Mr/Ms Betta. This is a definite danger to them as if they get stuck and cannot get to the surface to breathe they will drown.
They NEED to breathe once in a while through their Labyrinth organ on their head and if they cannot they will drown as we would underwater too long. So will gouramis and other Anabantoids blessed with the Labyrinth organ atop their heads.
While on this subject, any fish with these organs should never be treated with any medication in an oil base. These include the Melafix, Pimafix, and Bettafix (even though they say it is made for bettas) types. These are wonderful saving meds for other types of fishes but will eventually destroy the Labyrinth organs of the Anabantoid types and cause them to cease functioning and the fish will die a horrible death. As I have said though they have saved many other types of fish so I am not saying not to use them at all, just not on THIS type of fish. Learn to be label readers when buying betta products and keep the surface area clean, no floating plants or toys for the betta.
They do appreciate a tall plant in their tank as a resting place at night though. It is something you can do to be kind to your betta. While bettas love real plants, silk ones are appreciated too. Plastic can tear fins. But if you get a plant that reaches within an inch or two of the surface, it will save you tired little buddy a lot of swimming at night when he has to wake to swim to the surface to catch a breath. They all have to do it and it is much more distrubing to their sleep to have to do it from the bottom of the tank than from a tall plant. Just consider it a loving gift to a sleepy little friend. It may take them a few days to get the hang of resting there but once they do it will be their favorite bed.
Diablo even rests there during the day a lot as his fins are heavy and swimming with them is tiring I am sure. Rosetails have very heavy finnage and need to rest frequently and you will find that the more finnage your betta has the more resting he or she will do.
Enjoy your babies for that is what they are. They are much closer to you as their owners than any other fish you could put with them. They have been alone since they were found to be male and I am not convinced that they realize they belong with other fish. I am not even sure that they know that is what they themselves are. They relate to us more than any tank mate!
They are darling fish and I have loved everyone of them I have ever owned.
Rose
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