2 fx5's is well more then enough for a 180g. Overkill and waste of energy imo. I run 1 fx5, rena xp4 and a 13watt Uv sterilizer on my 225g and the water parameters are always perfect. I would go with 1 Fx5 and 2 HOB filters(AC110's). Save u energy in the end and if your FX5 all of sudden dies at least your AC110's can handle the bioload for the time being til u fix the fx5.. You go with 2 and both dies u'd be screwed...