Betta fish dying!!

[Ashyxiv]

From yesterday my Betta fish seems very weak. Looks almost dead and I have no idea what's going on. His tank is a 5 gallon tank and lived very actively until yesterday. He didn't eat from last Saturday and saw very weak movements and staying in the same place for too long. Today morning I thought he's dead but turned out he was alive but in a huge pain. I want to know the reason for this behavior since this can't happen again. This Betta's tank is the middle tank in my table and one side I have my marine tank. Last Saturday I checked the salt level of my marine tank and mistakenly hydrometer fell into this Betta's tank with salt water in it. I thought this can't be a huge issue and still I did a small water change. Some days ago I added 2 guppies to this Betta's tank and he killed one guppy overnight. Those are the only 2 'special' things happen to this Betta. Please share your ideas!! Thanks in advance.

 

[SaraB]

What are your water parameters? What is your water change schedule? How old is he? What is the temperature of the tank? What are you feeding him?
You can try feeding him something different like blood worms or daphnia.

Bettas with guppies is always a bad idea