05-01-2009, 12:05 PM
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Ah ok. The thread wasn't posted in the brackish section so I assumed it was a freshwater tank. With brackish water fish acclimation is especially important. "Brackish" can mean anything from very slightly brackish to nearly full marine conditions, so it's always a good idea to get a salinity measurement from the store where you're purchasing the fish and try to match those conditions at home. If you already have an established brackish tank that is significantly more or less salty than the tanks at the store, you should consider setting up a quarantine tank* with the same salinity as the store's tank. That way, you can introduce the fish without the risk of osmotic shock, keep it quarantined to watch out for disease, and slowly adjust the salt level in the quarantine tank to match your own brackish display tank. Then, you can transfer the fish to your display without problems.
It could also be that many fish stores don't really treat their brackish fish right. Even though you're keeping them in brackish water, the tank at the store could very well have been fresh (as it's cheaper) and thus moving the fish straight from fresh to brackish could have been a big shock. Doing so could have weakened the fish's immune system enough that it could have gotten sick and died after a month.
I'd say a 125g is a good minimum size for Colombian sharks. Beware that it likely will eat your mollies once it gets some good size on it.
*Actually, quarantine tanks are a really good idea in any case, it's just that they're especially useful when dealing with brackish water fish for the reason I already mentioned.