I think he means standard 55. IMO a 55 works fine till 12", maybe slightly cramped at 10, I have a 10" del in a 55 that is seemingly pretty healthy and active.. I do plan to upgrade. Just make sure you have lots of hiding spots and coverage, yet free open swimming space. And 520gph is 9-10 turnover rate, which is what one should be aiming for. This is average filtration on this site, doesn't mean you can over stock.
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