Siamese Fighting Fish On The Verge Of Death

I have had a steady fish tank now for two months without any casualties, in my tank was a range of different Gouramis two couples so four to be exact, a Siamese fighting fish and some mollies. However on the introduction of one more couple of Gouramis one has died and my Siamese fighting fish is on the brink with his tail in terrible shape, you can see in the picture. My tank is 450L and have close to no fish in there along with a couple of plants so I can't see how space could be a problem. Is there anyway I could save my fish? WHY IS THIS HAPPENING?

Gourami & Betta as a rule don't get along & keeping several male gourami together is imo a recipe for disaster as once they start to mature territorial battles will ensue.
I would separate the Betta though tbh he doesn't look like he'll make it, & I'd rehome most of the male gourami.

Judging on your discription, I'd say the new Gourami brought something in with him.
+1 on the post above, BTW.

+1 to what lillefishy said

What are your water parameters? You say the tank has been steady for two months without any casualties. What fish died and from what?

 +1 bettas should be on there own and the male gouramis should be reduced

My water parameters are good I've had them tested but how many male gouramis can I have in my 450L?

Who tested them? Your LFS? Did they use liquid tests or strips? What were the exact readings?