I think it says it's below 2 ug/l (not mg/l or at least that's what I see) which is way within safety levels. So there's barely any copper. Actually, if there was, they wouldn't be sickThe possibly significant number on your test is copper. <2mg/L
They'll do a lot of done daily for at least a week. Whatever it is, is still affecting new fish and is still in the tank. One week water change won't do a thing to a sick tank.I do not think 50% water changes will do much.
The reason I am suggesting big water changes on the big tank is to prevent spread of the disease to more fish. A sick fish as in losing balance is already severely sick and overwhelmed when this happens. And it's normally due to internal bacterial infection most likely and possibly parasites but they don't normally lose balance due to parasitic infections although the fish may still have parasites and bacterial infection at the same time due to the parasites. If kanamycin sulphate doesn't work in 6 days then I'd try a broad spectrum antiparasitic. As Mike suggested, flubendazole maybe worth a try.When solo was sick, we were doing daily 50% WCs in his 10g hospital tank.
Also, as it's the only tank affected, possibly the chemistry of the tank has shifted in a bad way so doing large water changes in a row will shift it back to healthy levels.
Your Ph you say is 7, comes out of the tap 8.2. Can you get a glass of water, test the ph then leave the glass to stay overnight, test again and tell us the results?
I've never used Triple Sulfa. They don't sell it here but here it is what it's for:
Sulfas are considered all anti-bacterials (antimicrobials).
These drugs are bacteriostatic, meaning they inhibit the growth of the bacteria but do not kill them.
Sulfas are generally most effective against aerobic gram-negative organisms, and occasionally effective against anaerobic gram negative bacteria, but are not reliable against aerobic gram-positive bacterium.
A relatively broad spectrum antibacterial medication; for fin and tail rot, mouth fungus and clamped or collapsed fins, Columnaris (mild to moderate infections ONLY), and hemorrhagic septicemia (although not effective to Aeromonas infections of the gut).
Considering your loach was losing balance, if you get another one like that I'd go straight for the kanamycin sulphate as it will treat internal bacterial infections unlike the triple sulfa which is what your loach most likely had. As Mike suggested, the sulfa may have had some effect but not quite enough or not for all the bacteria the fish was infected with. If the kanamycin works, the fish will start staying upright within 24-48hrs and should resume colour in 4-5 days but you need to finish the full treatment at full dose. Kanamycin doesn't normally affect the biofilter and is pretty safe with just about any fish or invert.
That's a good practice but if you have multiple filters cleaning one thoroughly won't make a bent. I wash my media under the tap when cleaning an individual filter. But that's off topic.We always clean filters individually and spaced out by several months, and we never do a thorough clean or replace the media