04-14-2008, 08:51 AM
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well, I think in general if they look similar (color, shape), AND they are known to be aggressive fish, then they will have problems. But I'm sure you are right on the point of it depending on which species you're talking about, as what I said above can't hold true for all fish. But I think its based on the fish thinking that it will have to compete with a similar-looking fish for food, as it probably eats the same things if it looks similar.
As far as a Midas Blenny and the Watchman Goby, while I don't know anything about the Goby, depending on the tank size the Blenny probably wouldn't cause any problems. I've read that territorial/aggressive behavior in Midas Blennies only comes out when kept in a tank smaller than say 30 gallons with another Midas Blenny (if they feel "cramped", they'll snap at each other). Though come to think of it, my Midas Blenny stays away from the bottom of my tank, usually swimming in the upper part and sleeping/resting in holes in the rocks... doesn't the Watchman Goby spend most of its time on the ground (especially if paired with a Pistol Shrimp)? If so, it seems like they would rarely cross paths, so you'll probably be fine.
Interestingly enough, once in a blue moon my Midas Blenny will sort of snap at my Regal Tang (90 gal tank), although otherwise they appear to be friends. They share the same little space in some rocks when they sleep or hide, they often swim together even though they've got plenty of room, etc. Its almost like he likes to horseplay :) The snapping never lasts more than a couple of seconds, and then they're back to being best friends.