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I plan on getting an O. bimaculoides. I'm going to house it in a 50 gallon tank, but my question is, does the sump have a required size? Would a 10 gallon sump be adequate, or do I have to have a bigger one?
the bigger the better IMO. I have a 55 gallon and I'm using a 20 long sump plus a 5 gallon refugium on top of that....and it seems to work extremely well. The more water you've got flowing through everything, the happier the octopus will be...and the water will be easier to keep at the right levels.
Most 50+ gallon tanks can accommodate a 20 gallon sump sitting under it. It doesn't have to be expensive....I bought my 20 gallon at a local pet store, just a plain glass aquarium.
I would use a larger tank than a 10 gallon for a sump. Main reason is that if the power should shut off the tank will drain into the sump until the suction is lost in the overflow system. This will be added to the amount of water already in the sump, and you don't want it to overflow.