I use a mixture of water and baking soda in an empty bottle that I keep in the bathroom and every time we use it we pour a little bit into the cup.

And it keeps it clean and without bad smell and the most important thing is that it is ecological and does not contaminate the water… I also recommend it…

Note: when we wash the bathroom we make the same mixture but with a little vinegar, too, we pour or spray it everywhere and let it act for 5 minutes and then brush and remove with water (it smells a little strong but everything is very clean and it is ecological).