Lymphocystis?

We're having a bit of a pond disaster at the moment and I'm hoping for some
tips. We had 10 goldfish/shubumkins in our pond but gradually they have all
died over the past year one by one. We now only have one left. The pond is
about 5 feet wide and 8 feet long and ranges from 3 -5 feet in depth. It is
filtered and ammonia and nitrite are 0, PH 7.5. Tonnes of frogs are present
as are tadpoles and all of them seem fine!
All the fish have developed white nodular lumps (just a couple each) which
have eventually turned into sores. The white lumps look just like pictures I
have looked at of lymphocystis.The last remaining fish seems quite happy
but has one of these lumps on his side and one at the base of his tail. He's
lasted a year like this so we don't really want to cull him, but we also
want to restock the pond at some point, especially as it's now spring so is
a good time. I don't know what to do. My husband wants to add a couple of
new fish and see what happens but I don't really want to do this as they
might catch whatever it is too. I also don't really want to drain the pond
and start again as I don't want to risk harming all the other wildlife in
it. I've read varying things about lymphocyctis. Some say it's fatal, others
that it's rarely fatal. If the latter is so then why have all my fish died?
I don't know what to do!
Any advice?