Partial water changes once a week is fine when the tank is fully cycled, but yours is not yet. In order to prevent damage to the soft tissues and internal organs, you need to keep the levels as low as possible. The two options you have for doing so are either 1. Keeping him in the tank and doing 50% water changes every 12 hours or 2. Keeping him in a tub and doing 100% water changes, plus adding ammonia to your tank every 24 hours
I personally recommend option 1 as the easiest because for option 2, you need to be sure that the tub remains an appropriate temperature and has adequate oxygenation while also adding ammonia into the tank itself in order to artificially continue the cycle.
With both options, you really need to be on top of those water changes, do regular testing, and be ready to intervene if something goes wrong. There are lots of cycling guides on this website and online, but I can also give advice if you have further questions.