Thanks for your quick response. I have researched this intensively but made few changes.
Water top-up was only in Jan. when my foot was broken. I left both covers open to increase evaporation so top-ups were well oxygenated. I checked the parameters weekly and they remained excellent.
Since then 15% weekly water changes, heavy UG vacuuming, and two 50% water changes. (copperSafe replaced religiously for the first month -- February. Needed religiously after each water change since then, including 15% yesterday. Level is 1.5.)
My ammonia and nitrite readings have been zero the entire time and nitrates well into the safe range. I've checked readings weekly before and after water changes/symptoms. (My filtration is massive -- 75-gallon tank with a 90-gallon eheim canister, a 60-gallon HOB, and a 24 x 18-inch under-gravel with one uptake and one power-head. The eheim was a Christmas present -- the only change and six weeks before symptoms. I have cleaned the media three times with no change in parameters.)
All fish experience the symptoms within a day -- even the angel and plecs.
Extremely rapid gill action takes about 24-hours to develop fully and subsides gradually in roughly the same time. (To be clear, there is still coppersafe minus the water change volume at all times.) Flicking takes place on day two and very occasionally with the siamese algae eaters and featherfin at other times.
Regards XXXXX XXXXX lost a clown loach before treatment (two remain despite quite a battle). The lost one went head up as did the other two - not at surface - with the heavy breathing. Then featherfin got a little grey and flicking was universal so I suspected velvet. Since then I've only lost an ancient platy female. She didn't die until this week. Probably natural.
BTW, my overstocked 20-gallon is completely unaffected.