Should I use lasix?

Question
My rat has fluid in his lungs, the vet prescribed him amoxicillin and he has been on it for over a week. I am more concerned about the fluid in his lungs, he can't breathe, but the vet says giving him lasix is too risky, something about him losing too much electrolites and bodily fluid if there wasn't enough fluid in his lungs. Isn't that what pedalite is for? I've been nebulizing him since after his 11 days on amoxi and he doesn't seem to be breathing any better after 3 doses (5 minutes, 4 minutes, 8 minutes 4 hours apart). Keep in mind he hasn't been able to breathe properly for all this time (11 days), everyday looks like his last. Is giving him the lasix advisible?

Answer
You didn't specifically state this, but fluid in the lungs leads me to believe it's pneumonia, and it sounds as though it is in an advanced stage, for which the prognosis is not very good.  

Now, let me re-state what I state in my profile, that I am not a vet nor do I have extensive medical knowledge.  However, I will try to give you some guidance.

I personally have never had first hand experience with Lasix, nor have I heard about it's use by anyone within my rat community.  However, I did find in a rat medical guide that it (and I quote) "May be helpful in rats with severe cases of pneumonia".  Here is a link to the article where I found this, but note the section that discusses "Adverse Reactions".
http://ratguide.com/meds/urinary_tract_agents/furosemide.php

In advanced stages of pneumonia, a medication called Prednisone, which is a corticosterioid (given in addition to an antibiotic) may help reduce inflammation in the lungs, helping a rat to breathe easier and reduce gasping attacks.  It may allow a rat to live an extended life more comfortably.  Prednisone should be given at a dosage of .25 to 1 mg per pound once daily. Normal use of prednisone would be only for a few days.  

Another drug helpful in severe cases of pneumonia is Aminophylline, which relaxes and expands bronchial tubes, helping a rat to breathe easier.  The dosage is 2.5 to 5 mg per pound orally or injected twice or three times daily.

My recommendation is to get a second opinion from a different vet who is experienced in treating rats.  Bring with you the information I've given you above to discuss the options and his opinions on each of them.  If you cannot find another vet, then bring this information to your current vet to discuss...perhaps he is not aware of some of the medications I named above and can do some research or contact some of his colleagues for advice.

I'm so sorry I can't help you any further but I wish the very best to you and your rat and hope that you can find a vet who can help your rat overcome his illness.