Jack and Diane

I had two jewel cichlids and 3 fruit tetras in a 29gal tank. They were thriving and colored up as red as can be weeks after I got them. I wanted to make more decor for the tank so I bought river rock (which I boiled and dried). I built a rock wall which turned out beautiful. I used non toxic glue gun glue and silicone to put it together. Thinking everything was perfectly safe I added the decor. The next day the tetras were all in a plant dead and the cichlids turned gray, as they do when stressed, I quickly moved the cichlids to the quarantine tank. I added Melafix and removed the carbon filter. I checked the 29g's water and the parameters were fine, even the ph? The larger cichlid died the next day and the other still in quarantine. He hasn't eaten but seems to be getting color back and moving around a bit. I threw away my decor and cleaned the gravel. I filled it half way with water from a good tank and the rest with primed tap water. The tank is fine now and housing a few tiger barbs. I just can't figure out what went wrong. I adored my jewels and I haven't had a fish death in a long time?