Originally Posted by somewhatshocked
If you need solid flow, a 2217 won't really do much for you. I wouldn't use that filter on anything larger than 20 gallons. Should probably go at least one step up if you stick with Eheim in a tank that large.
I disagree. I have a 2215 w/ a 2217 impeller on a 20 long. It has way more flow than I need, though I like a lot of flow. I used it with the stock impeller on a 29 gallon with no issues, I would say a "normal" amount of flow for a high tech planted tank, not excessive, which I like. I think a 2217 would be perfectly adequate for a 40 gallon tank. I personally would want more flow but it would be fine.
My brand vote is Eheim. But, as stated, most brands are good. Not sure how much flow you want so pick a size. At minimum for low tech you are going to want the water to turn over 3x per hour. Many of us go well above that to 10x per hour. Some go really high, like 15x per hour.
What that means is if you have a 40 gallon tank, for 3x per hour, you need a flow rate of 120gph. Real, not advertized because most are advertized high. They aren't necessarily lying to you, they just rate it level (not under a tank) without any media inside. That's just not accurate for how we use them, not sure why manufactures do that.
Sorry if that sounds convoluted but just try to get an idea of where you want to be flow wise, with 3 being pretty low, 10 being high, and multiply that number by your tank size, which in this case would be 40.