Over the last few days we have noticed our puppy's (he is 19 weeks) breath is a bit fishy!!!  I am sure it wasn't like this previously.  I haven't changed his food.

Has he been licking his bottom a lot?  It could be his anal glands, has he had loose motions at all?  Are his poos well fromed and the kind you can roll on a shovel (normal) or do they have to be scraped up?  A normal motion should cause the anal glands to empty properly.  A dog can also release the anal glands if it is frightened, or overecited, and you get this horid fishy smell.

What is the protein source in his food, if it contains fish he may well get fishy breath.

Could be his baby teeth. When our boy was a pup he developed the most awful breath (I remember posting a message like yours at the time !). We could not find a cause until his puppy teeth started to come out. When he woke up one morning I found one in his bed and don't ask me why, but I sniffed it !. It was dreadful and I guess it was the rotting tissue/teeth that caused it. Maybe it could be the same with your pup - has he got any loose teeth yet ?. If one comes out give it a sniff !.

Val

I think that may be what it is.  He has lost a few teeth but I checked in his mouth and one of his back teeth was a bit off colour and very loose, so I helped it along by brushing his teeth.

No he hasn't been licking himself alot.  Also his toilet is pretty solid.  I did check in his mouth and found that one of his baby teeth was a bit off colour and very loose, so I brushed his teeth and this helped it along and it has now fell out.  I think may be this was the problem, but I will continue to keep an eye on it until he has finished teething.

I also think that this may be the cause. Mine was excactly the same until his baby teethe dropped out. I used to give mine the Pedigree Denta Rasks, to help it along, and clean them at the same time, this helped a bit also!