I paid £250 pound once for a breeding pair of solid blue cobalt discus which were killed by my nephew/ god son by pouring vimto into the tank. My missus phoned me saying the fish were on the side when I was working away so I drove home 2 1/2 hours from Manchester that night and did a massive water change and see what was going on. The nephew swore he hadn't done anything but I noticed the side of the tank was sticky, wet my finger and had a little taste (probably not a good idea could have been anything 😳 ) Tasted like vimto, asked my missus and he'd had a glass. One confession later he admitted given them a drink. 😡
I don't know what chemical is in it but discus didn't like it at all. One fish died the next morning and the other the day after that. Ouch! Never again.