Columbian Shark Dying?please Help!

[ThisIsMyCareFace]

This is a re-post from fish emergencies, it was suggested to me to post it here for more input:

Hi everyone, I am new here, the old fish forums I used to belong to have been deactivated, seems the entire site is down forever. I have a serious problem, I have a 80 gallon brackish tank with 3 columbian sharks and one white convict cichlid, they have all been living in this tank for almost a year, and i've had all of them together for around 2 years. My problem is that one of my columbian sharks is not eating, and hasn't eaten in about a week, I am very worried, the other 2 sharks are swimming around happily and eating plenty, and fat, all the other fish are fat and appear healthy. I feed them a variety of food, and they eat it all, i feed them tropical flakes (they go through alot so it is always fresh), they eat shrimp pellets, slow sinking crumbles, brine shrimp (the dry cube kind), and sometimes live mealworms. The shark that appears sick swims at the front of the tank and his/her skinny belly is quite clear to see, and doesn't even attempt to eat at feeding time as the other fish pig out heartily. the sick fish also swims to the top of the tank and gulps in air and it comes out of his fins in large bubbles. Please someone help me save my beloved shark, I have raised all three of these sharks since they were about an inch long and it will break my heart to see one die. There is no marks on the fishes skin at all, and aside from not eating and rapid breathing and looking skinny, looks like the others. I am definatly due for a 30% water change but i have put this off for quite sometime due to a broken salt meter, or whatever its called. The ammonia and ph and temperature levels are all within acceptable range for these fish, and the cichlid doesnt seem to mind the brackish water at all. Please help me i am very desperate and i will be checking back every hour (unless im sleeping) to hopefully solve this problem.

Thank you-new to your forums-Levi

 

[nmonks]

Greetings and welcome to TFF.

That you have a convict cichlid alive in this tank suggests the salinity is low, too low for Colombian shark catfish. While tolerate of a range of salinities, once they get above, say, 10 cm/4 inches long, they do start acting weird if the salinity isn't high enough. A good starting point with juveniles is SG 1.005 at 25 degrees C, while subadults and older fish will need around SG 1.010 at 25 degrees C, and ideally anything up to marine conditions. It goes without saying that maintenance at SG 1.010 would kill a convict cichlid quite quickly.

I'd also like to know what you mean by an "acceptable range" in terms of water quality. Just to be clear, this means 0 ammonia, 0 nitrite, a pH between 7.5 and 8, and a hardness of at least 15 degrees dH.

Cheers, Neale