Thanks for the replies guys. My Royal Gramma gave up after approx 3 hours of deep breathing on the bottom; my green goby was actually okay and not affected (thought it was), and my yellow clown goby was breathing extremely fast, in the same spot, for almost 5 hours. This morning, I'm pleased to report that he now seems absolutely fine. I'm really pleased about this, as he's now the only surviving fish from my original tank. I just need to get to the bottom of this now before the next waterchange!
Yes, I use the same salt I always have. I guess it could have been contaminated somehow (perhaps a bad batch?); it's stored in an air-tight container under the sink, no damp or anything, dry, room temperature. Is there any way I can test if there's a problem with the salt?
The house is approx 20 years old; not sure about the piping. Wouldn't anything like that (e.g. metals) show up in the TDS
test thought? If not, again how can I test?
Pat: I'll run carbon now, thanks.