I have two, the newest addition to my tank 2 days ago. After I got home I was reading and found that one source suggested 6 or more as they do better with more than with fewer. I have 4 more on order now. The first day I added them I noticed a couple of hours later that one was missing. I seached all over the tank then my overflow area. Nothing, thought maybe my goby had a meal of chromis. I later went to change the filter and there he was, swimming around on top of my filter tray. Must have been a wild ride for him to get there. I would call it a e-ticket ride! He hasn't been near the overflow since. Is better for me as now they swim out front and not hide in the rocks.
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If you can read this thank a Teacher, because its in English thank a Veteran.
110 Gallon FOWLR, 25 Gallon sump/refugium.