keeping floating plants alive over the winter

Hi,

I got rid of my aphids, thanks for the help. But I have some more questions.

I have my floating plants in a tub in the basement they seem to stay alive,
(after I got rid of the aphids)
but the aren't GROWING, just living there. I think they need fertilizer but
I don't know what to give them, or put in the water.

Any ideas?

Thanks, Joy