BIG canister filters

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I recommend Eheim.

For a 180 gallon tank I would use 2 Eheim 2028's. Better yet, go with the 2128 thermofilters so you will not have to place a heater inside your tank.

These are expensive but you can save a lot of money by buying from a U.S. online site.

Eheim are:

-reliable
-quiet
-low maintainence (I clean my Eheims twice a year)
-sturdily built
-self priming
-able to be used as a CO2 reactor and filter at the same time

I have four Eheims, I would not choose another brand.