You set up the tank. You use new water because, why not? It's clean and there is no bacteria in your water anyway. Just run the tank for 24 hours first to aerate it.

So now you've gone through a cycle and want to save it? No problem.

option 1: take the media out of the filter and put it into the new filter – done and done. Transfer the media, transfer the fish.

option 2: if you can't fit the media into the new filter, run both filters.

As far as your substrate is concerned, don't worry, I've done the transfer without issues. Just set up the new tank with sand.

Sand is awesome and I would go with nothing else now. If you need advice on maintaining it easily and quickly I can help out – but one thing for sure is that it provides a much cleaner tank over the long term then gravel and is much better for burrowers, sifters, and bottom dwellers in general.

Fish in general do not keep a tank clean – that is your job. bottom dwellers help eat uneaten food, certain scavengers can help with algae, but all donate their own mess to the tank. Don't ever by a fish to 'keep a tank clean', it doesn't work like that.

Good luck.

PS: talking catfish are groupers, I keep 6 of them and they are very social fish, I would never keep one alone.