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I am building a canister filter and what I want to know is, does it matter if I have the dirty water flow from top to bottom or bottom to top? Is there an advantage in doing on way or the other?

Jerry: You should post more details about your design. Are you using the popular 5-gallon bucket design? I'd say the answers to your questions lie in the details.

Personally I opted for a open sump filter, but the benefits of a canister filter are definitely great (mainly the not needing an overflow box).

I am building a second canister filter using a 5 gallon bucket for a new tank I'm gearing up for. The tank has a plug on the bottom, so I will drain from that, which will hook up to the bottom/side of the 5 gallon, and then flow out the top. The pump would sit either on top of or next to the bucket.

I don't want to make one that big, this is for a 10 gallon tank. I want to make one like the DIY filter sticky above. Eventually I will upgrade to a bigger tank, then I will probably just save up and buy a filter.

Ah!
Well, for those little guys, I'd recommend feeding in from the bottom side and going out the top. Have the inlet feed into a T fitting, with a spray bar extending out either side. This will help distribute the water more evenly through the filter media.

That is a good idea, is there any reason to do it like this or just because it is easy and pretty much the norm?

It just makes sense to me, I guess. It seems commercial canister filters usually have both inputs on top, but doing it this way makes things simpler, and gives you more room in the filter for media.

I agree, now I must build.