I have the 205, 306, and 406 units. Including the intake and discharge hoses they measure 12-1/2", 16", and 20" accordingly. The FX6 model measures 22" tall without the hoses and 17" or 18" wide including the pump. You only need about an inch to remove and reseat the top/motor of those models mentioned but you also need some area inside to set the head down since the hoses stay attached to the unit.
Canister units will do NOTHING to control any water parameter spikes. The only way to control those are thru excellent husbandry, proper selection of compatible species and NOT overstocking tanks. (Although that's not related to your question, I just thought I'd supply those we'll known facts for others.)
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