09-17-2009, 01:03 PM
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Assuming the fish in question is Arius jordani, common names Columbia Shark or Silver-Tip Shark, it is a brackish water fish when mature as Twistersmom mentioned. It is also large, growing more than 12 inches, and needs a 4-foot tank at a minimum when young. Smaller tanks (not saying this is a problem with you) will probably cause growth problems internally.
With respect to success in fresh water, mollies, I note that your pH is 8.2 and that probably has something to do with your luck so far. These fish require harder, alkaline water, and that means there are more dissolved minerals and salts of various types in the water than there would be in water with a pH of 7.4, and water with a pH of 6.8 is certainly not acceptable for this fish. Thus, the fish you have may be "OK" in your 8.2 water simply because it has so far managed to make do with the harder mineralized water. The long-term continuance though is not so certain, as I'll come to below.
And catfishtabbi, this is another issue for you, with water that is slightly acidic to slightly basic (alkaline). This fish, when young, requires harder water.
A final comment on Twistersmom's pertinent comment on fish being more prone to disease due to water parameters. This is absolutely true. It is a scientific fact that fish have a certain "preference" for water parameters, and each species has evolved over millions of years to "live" in certain water. Evolution is an on-going thing in nature, and over time fish continue to evolve to adapt to changes. This is one reason why decades of tank-raised fish can adapt to different water parameters than their wild counterparts. But there is a limit to such adaptation short-term, and differing water parameters also takes a high toll on some fish more than others.
A fish has a complex internal metabolism that involves adjusting the ph of its blood to equal that of the water in which it lives, feeding its cells, and maintaining its immune system. When it has to work harder to do any of these things, it causes stress. And just as with humans, stress brings on various diseases. The fish is literally weakened constantly because it is spending considerable effort to match its surroundings, and parasites--which are frankly always present in our aquariums--will take the opportunity to attack and the fish will be unable to fend them off. Salinity and ph play a significant role in this, because they affect the internal workings of the fish.
To sum up, for the long-term health of this fish, it should be in a large aquarium of brackish water. It is also a shoaling fish, and prefers being in a small group--another problem with tank size.