it started a month ago with my 18 inch arowana.. he was big and healthy ive had him for about a year..one day he quit eating two weeks later he died...someone told me to put melifix in the tank so something wouldnt spread to my other fish so i did. The melifix killed my 14 inch black ghost knife in ten minutes..now its a month later and my big healthy vampire tetra is doing the same thing as my arowana and is goin to die within the next couple days...I just dont get it i have very clear water its a 180 gallon tank with very good filtration and flow with power heads and two fx5 filters. when my arowana quit eating i did a 25 percent water change and the filters were barely even dirty after one year of use. The fish i have left are a foot long teacup ray, a 17 inch polypterus enlicheri, and a 9 inch sand goby which are all healthy and eating... should i just do a gigantic water change or what cause this is a nightmare and i have alot of money invested in these fish...