Hi Roediger,
Happy to hear the good results so far. Hope it continues. Yes, dim lights reduce the growth rates but so what? You don't have to deal with algae. That's a great tradeoff.
As we mentioned before, we try to follow the 10X rule. I had a look at Big Al's and it lists that filter as having a flow rate of 60 GPH, which would be good for a 6 gallon tank.
Again, if the lighting is kept very low and if your distribution scheme is good and if your CO2 method is good then that filter would be fine.
If it were me though, I would rather get a filter that comes closer to the rule. Anything with 80-100 GPH would be better if you can get it at a good price.
It's not clear to me if you have an aux pump in that tank as you stated "...my 10 gal with good flow..."
Does that mean the 10 G already has good flow? If it already has good flow then the Eheim will be fine and maybe even OTT.
the 10X rule applies to the total pumping power rating in the tank, so if you had a wavemeker giving good flow rating in the 10 G already then as I said, no issues at all.
Cheers,