Lily Problem

The lily I bought for my new pond a few weeks ago (Nymphaea Carnea) has
had it.
The leaves developed black patches and one by one the leaves rotted at
the base and detached themselves. I took the tuber/root out the pot and
found the top to be soft.
I,ve tried re-potting it after cutting the soft area off and put it
in a bucket rather than the pond but I'm not too hopeful.
Two questions.
Googling for Lily problems I found mention of root rot a fungal
disease. Does this sound like what I've got and if so what steps can I
take to prevent it happening again.
In retrospect I suppose I should have planted it in a bucket until
sure it was healthy. Is this worth doing for any new plants. If so for
how long.
Is it a possible water quality problem. The water is clear. I
haven't tested it but the fish are all fine. Is there any levels ie
salt nitrates ph etc that would kill lilies.
Iain