How are a qt and a hospital tank different---or are they?

Difference between a qt and a hospital tank...
There need not be. What describes a hospital tank can be a qt, IF you're willing to do very frequent filter changes, and you can do it as one tank. Follow with me. Understand the trade-offs.

A 'cycled' qt relies on bacteria. You can go as far as sand, rock, etc...or just sponges with bacteria. You put the fish in, you watch fish for four weeks. If you have to treat, you then move the fish to a hospital tank. And if it's a sand-rock fuge and ich was involved, you do not use it again for 12 weeks [time for the ich to starve and die].

You cannot medicate in a cycled qt without risk of dieoff of bacteria, which affects water quality and oxygenation, at a time when the fish, due to ich in the gills, may not be breathing well in the first place. I won't say fish can't survive despite this, but you certainly can't expect it to set up as a bacterial-run tank after having been coppered, or dosed with an antibiotic--- Most sandbed-type bacteria ARE gram-negative, and most infections are gram-positive, which is why some antibiotics can avoid killing your sandbed, (though some gram-negative-safe antibiotics will still ding it ) but if you use a broad-spectrum antibiotic, guess what? It takes out both kinds of bacteria, gram-negative and gram-positive. So whether copper, which kills all sorts of bacteria, or an antibiotic, which is an either-or, you are just better off moving everybody out to a tank with NO biological filtration to die off and adversely affect the oxygen level.
You can use hyposalinity in a cycled qt, but I'd still wait 12 weeks after the last fish has left it cured before allowing another fish into that qt tank, just in case the last ich is somewhere encysted in that sandbed.

A hospital tank or a transfer tank does NOT use a cycled sponge or sand or rock. You simply use a cheap disposable filter (pillow floss from the sewing store or fabric store is ideal) which you toss the instant you spot any brown stain (biologic activity) on it: you keep it microbe-free, so it won't cycle, won't produce ammonia---and WON'T provide a haven for ich-swimmers, who when they leave the fish, typically lodge in the sandbed or physical filter before reinfesting the fish. You can medicate here safely with just about any med safe for the fish. But two cautions: when you are dosing a med, you MUST have an autotopoff on the tank to prevent evaporation changing the dosage...do you see how? When the water evaporates it concentrates the med or the copper to a higher dosage than may be safe. If it gets too much water, if lessens the dosage. If you are using hypo, you also need the ATO, for exactly the same reason: hypo relies on low salinity killing the pest---but it doesn't work if evaporation keeps on saving the ich by RAISING the salinity every few hours. Whatever your treatment, an ATO keeps it both safe, and effective in dosage.
You also need oxygenation---really good oxygenation. The med is going to stress your fish to the max, and being oxy-short is going to make recovery difficult.

I hope that helps you understand the tradeoffs. If you're going to treat, you need to do more than just dump in a med and cross your fingers. Treatment itself can kill, by creating low oxygen, poor waste disposal, an overload of dead bacteria, or just an overdose or underdose of the medication or treatment.

If you use an uncycled qt, you can convert it to a hospital tank just by adding meds and the ATO, and you can clean it with vinegar and sunlight, ie, clean it and all gear, and let it dry out. Then it's ready to use again.

But an uncycled qt must be maintained scrupulously clean, with NO biological activity allowed to set up. The waste will be removed by the filter, which you toss very frequently. Hence pillow floss.

One more footnote: a filter with black stuff in it has CARBON, and carbon may pull your med from the water. Do not use carbon in a tank you're treating.


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