They appear to be the extremely rare and difficult to keep invisible zoos. 
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I retired and got a fixed income but it's broke.
Ed
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50 gallon FOWLR, 10 gallon sump.
130 gallon reef, 20 gallon sump, 10 gallon refugium.
10 gallon quarantine.
60 gallon winter tank for pond fish.
300 gallon pond with waterfall.
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