I've currently got a betta and a cory cat in a 2.5 gallon tank with no
filter and no plants. I want to move the betta into a 5.5 planted tank
and add a filter. I'll move the cory to another tank and get him some
companions, replacing him with an SAE.

Actually, I've already set up the tank and cycled it. The problem is
that the filter on it, the smallest Aquaclear, generates too much water
flow for a betta, even turned down all the way.

So I'm looking for a different filter. I could go with no filter at
all, but I tried that in the 2.5 gallon and it seemed to increase algae
growth.

The only thing I've come up with so far is a Duetto 50, but I'm
wondering if there are other alternatives. Suggestions?

BTW, the airstone driven corner/box filters seem to have quite a
turbulence effect from the bubble stream, so unless there's a way to
quiet them and still have them work, I don't think they'd be suitable.

--
Homo sapiens is a goal, not a description