Short answer.. depends...
There are several ways I've done it, depending on the application.
If you are using medications, you don't want a skimmer or carbon in the filter.
I've used
sponge filters with airline... you can cycle a new sponge in your main tank/sump and have one at the ready at all times. After treating illness or medicating, throw the old sponge away, as you've had a new one cycling in your main system. This is an economical way to do it.
OR use a power filter with
bio wheel or the like - take out the carbon cartridge. You can cycle a wheel in your main system, just by dropping it in the tank or sump... then use it for the QT. Don't return a used wheel to the main tank, lest you transmit something back into the main system. This works but it's a bit more expensive - however you can keep the same wheel in the QT as long as you keep some biological activity going (I suppose you can do the same with a sponge too...)
If you medicate, watch your params carefully as most medications can affect the biological bed, can reduce dissolved oxygen and you can experience low pH and/or ammonia spikes. Do water changes as necessary but monitor the amount of medication and adjust the dosing accordingly, or you may not be successful in treating whatever you are treating.
Use plastic decorations/plants/PVC pipes as hiding places for the fish, they need to feel safe but at the same time, the ornaments need to be easy to clean and sterilize.
Use no substrate.
HTH
Jenn
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Imagine Ocean

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