I have the Penguin BIO-Wheel 330 on my 30 gallon tank and it would be more than adequate for a 46 gallon tank. I found it online at www.thatpetplace.com
for just over $20. I just saw on their website that Marineland has just started shipping the Penguin 350B that is replacing the 330. I'm not sure what the differences between the two are. There is no mention of the new filter on Marineland's website. Here is a link to a review I wrote of the 330.
I hope this helps. If you are going to do a planted tank with CO2
injection then I would recommend going with a canister filter instead. The reason is that with CO2
injection you want to minimize the gas exchange at the surface and the 2 BIO-Wheels on the 330 are designed to maximize gas exchange to aid in the growth of aerobic bacteria.