Originally Posted by STS_1OO
A 2217 would not suffice for a 40g only if you are injecting CO2 and ferting regularly into the water column.
Otherwise, a 2217 is enough.
Instead of talking about flow, it's more prudent to understand why you need flow in the first place. It's not universally necessary or even preferable.
If you do need the high low, you don't need to get two 2217s. You can get a 2217 and a power head to increase flow. It's more mechanical clutter in your tank but much more affordable and less to maintain.
Also, there's no problem with turning your filter off and cleaning it. If your tank is established and not critically stocked, you'll be fine.
Two 2217-sized filters is a luxury on a 40G lol.
I don't fert or use CO2 regularly, so i think that a 2217 would be ok