I have a wooden bridge that needs wood preservative - does linseed/danish oil kill fish as there would be a certain amount of spillage?

I would say that if it is not part of a fishes natural environment, that it will more then likely kill your fish.

You need to remember, in tanks, things that typically would not harm fish in the wild, will kill tank fish.

Is this a new wooden ornament? I have read many places that putting wood in tanks isn't the best of ideas. It should be completely soaked prior to placing in a tank. Water naturally preserves wood... It might not look pretty in 6 months, but it should decay.

I'm thinking.. she is talking about putting a bridge over a pond? She said "spillage", can you take the bridge away form the pond to preserve it? It will most likely kill your fish.

I only read the first part :P I need to stop doing it.

Yes, if it is not part of a fishes natural environment, then it will more then likely kill your fish or make them VERY sick

Haha lucky, it's okay :)

Anything that can get into the water will kill, especially oil based products. Treat it and let it weather away from the pond for a while before putting it back.
P.S. Emily is right, I reacted before I read all post.