The iRobot cleaning agent is also for the J9+.
Half a bottle per 3 litre tank.
This is recommended by iRobot.

Hi Anonymous

Thank you for your question. The cleaning agent here can be used for all iRobot vacuum and mopping robots and therefore also for the Roomba Combo j9+. As already written by sreinhard83, we recommend adding about half a bottle to the 3 litre tank, but the quantity can vary as desired. In principle, the appliance can also be used with water only.

Other cleaning solutions can also be added, but care should be taken to ensure that it does not foam and that the nozzles and hoses are not blocked. Use with essential oils is strongly discouraged.

If you have any further questions or uncertainties, please do not hesitate to contact us.

Best regards,
Your iRobot Team

I couldn't find the information either. But I use Mr Proper for floors in a low dosage to avoid corrosion, and no problems so far! I also add a bit of essential oil, which gives it a nice scent ;)