i use a whisper internal filter 2-10i in my 10 gallon, hangs inside the tank, works good, is virually silent (can't hear pump or water-easy to sleep near) and makes water movement but it isn't rough. no dividers in the tank, current definately doesn't bother my girls. putting some floss in the cartridge will help slow down the water movement too.
I use box corner filters with carbon and floss for the 5 gallons (boys) with an inexpensive gang valve. none of the boys i've ever had have been affected by the filters, they don't mind it if the gang valve isn't even on and it has a current but the 10 gallon pump i use runs two 5 gallon tanks (thanks to the gang valve) but it probably depends on the betta. i bought the gang valves to connect the pump to two tanks - turning down the current was an after thought. pumps do come in sizes, make sure you get one small enough (my goldies are in a 30 gallon with a whisper external and a box filter/20 gallon pump to get rid of all the extra garbage since goldies are quite dirty - this pump could possibly blow a betta away in a small tank) the only thing i don't like about them is that i haven't found a pump that doesn't rattle and make noise. some do make less noise than others some are awful. from what i've bought it seems to vary by pump not even by manufacturer. these are in the living room so it's never quiet in there anyway and the noise blends in but i tried and couldn't sleep with one. my husband can sleep through anything so it didn't bother him at all
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if one of the pumps goes i'm going to try an internal whisper 2-10i with the 5 gallons and add more media and keep it closer to the water.
the 30 gallon tanks have the external whispers that hang outside of the tank and work like a water fall but i haven't tried those with bettas. i did notice some of these for smaller tanks have a switch that can turn down the flow but others don't. a switch could possibly make them work. the water does make noise but a water fall noise. no pump sounds.
reason why i picked these 2 filters for the bettas? i can get the cartridges in bulk online (http
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/www.bigalsonline.com/splash.html less than 1/2 price of my lfs, ships just about instantly and i get it in 3 or 4 days - there are many other good places, this is one i use most often) and they last a very long time once the tank is cycled (rinse cartridge in tank water to clean out extra gunk at cleaning time to get it to last longer) so it's really cost effective in the end that way. carbon and floss also last quite awhile. since i'm rinsing and adding carbon to the cartridge as needed since i use it quite awhile it isn't really simpler than the box filter. works about the same in the end time wise.