What Happened To My Float Plants?

[dingweding]

I have 2 tanks for my shrimps. One is hard water , where PH=7.8 , GH =15, KH=12. Typical london water. I have some water lettuces and Amazon Frogbit there. Because it is low light, they did not grow very fast, but they survived, esp the Amazon Frogbit , have many new baby plants.

Last week, i moved some of them into another relative soft water tank. where PH=7.2, GH= 10, KH=5. It is also low light, although a little better than the first one. I thought they might grow better. However, i found most of them died, the water lettuce all rotten, even a huge one more than 10cm. The Amazon Frogbit turn yellow as well, some root also rotten.
why is that? in this tank, i have peat, bogwood and some leaves release tannish to reduce the PH and GH. I thought the plants should like this. could this be the reason?

Many thanks

 

[Sanj]

Sounds like there is not enough nutrients in the water column...do you dose? Also if the water cabbage has gone rotten could be that condensation was falling on it?

 

[dingweding]

Sounds like there is not enough nutrients in the water column...do you dose? Also if the water cabbage has gone rotten could be that condensation was falling on it?

i do not dose, and it is low light. Because float plants are normally tough, that is why i put them in the 2 tanks.

And wot surprised me is, why in the first tank, they grow ok, but in the second one they rot?