The Dyson Submarine™ wet roller cleaner head has been engineered to clean effectively with just water. But if you decide to use detergent, please a choose gentle, low-foaming household anti-bacterial detergent. Do not use bleach or other harsh chemicals. Please follow the manufacturer's guidance on dilution for surface cleaning - and we’d advise against putting neat detergents in the clean water tank.
Depending on which floors I mop (oiled parquet, tiles or epoxy resin floors), I add a super small amount of cleaning agent. This has always worked without any problems. But you should clean the container after use, as well as the mop roller.