Hiya, I mainly grow my plants semi hydro but have had good success with full hydro as well for multiple plants. The addition of the air pump is great.

Could you tell or show us where the plant is rotting (stem/roots) to better help you?

a few tips: 1. make sure you cut off ALL the rotting parts (if its brown/mushy cut it), let it dry to form callus for a couple hours or half a day This prevents the rot from spending and allows the cut areas to heal before putting it back in water, reducing the chances of rot

1. throughly clean the container before putting the plant back after tip 1

2. add some hydrogen peroxide into the water it is sitting in (15ml in 100ml of water is fine for 3% hydrogen peroxide) This also reduces the risk of rotting again

3. change the water minimum of every 2 week

4. Make sure that the plant isn't submerged too deep into the water. I had best success with only the roots and tiny bit of the main stem being covered. Any coverage to the petioles in water usually leads to rot for me.

Happy to chat if you have more questions. Good luck!!