03-07-2014, 06:17 AM
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They usually have one huge filter connected in a back room to all the tanks. But they also do maintenance like a lot of people, with the vacuum, I've seen them cleaning with them, vacuum, the manual ones into a bucket, and a hose connected to a sink running water back in.
As for aggressive fish living together? It's usually cramped and sucky in petco tanks, they are either too ugh to want to fight, water too hot or cold for them, sickly, or they've just given up because they're so cramped, a lot of more aggressive fish wont fight when over-cramped, this does NOT mean it's healthy or good for them so do it too...Cheap way to keep them, many are usually so stressed they die after a night or two in a new tank, especially if one picks at them.
Same with the goldfish, if there's not a huge deal of room to swim, why would they try? You take those fish home, put them in a proper tank and yeah, the cichlid comes out in them. It's how they're -supposed- to be, and if things aren't proper in the group, one will make it proper for you.