I agree with Astroguy...you need some test kits, and the strip kinds really are not accurate.
What food are you feeding the betta? I give mine 3 or 4 pellets a day. I feed Marineland Bio-Blend Betta or Hikari Betta Gold. It depends on the size of the betta, too. The bettas I have now are pretty big. The ones I used to have were smaller, and I didn't feed them as much. I also fast my bettas one day a week.
Here is some medicated food: http://www.drsfostersmith.com/produc...1&N=2004&Nty=1
But, if you've had the betta for 18 months, in a tank by himself, I don't think he has external parasites. I guess it's always possible that they harbor internal parasites, or he was infected before you got him.
Try to feed him a pea -- it may clear his system if he has a blockage and is constipated. Cook the pea (microwave or boil) and then let it cool. Don't feed it to the betta as hot as we would eat it! Then peel off the skin and see if the betta will eat the "meat" of the pea. I've found that frozen peas don't mush up and mess up the water as much as canned peas do. Feeding him a pea may help to clear the bloating. Don't give him any betta food for a few days to let his system rest. You could give him a pea for a 2 days in a row, then fast him. Of course, it's easier to pull a pea out of what you're cooking for dinner than try to cook a single pea!!
See how he does after the pea and fasting treatment. You may need to try some medicine. I've used Jungle Fungus Eliminator successfully when my betta was bloated. I used it for 6 days and he got better.
If these things don't help, you could try the medicated food.
How big is his tank? Ammonia and nitrite should be 0, and I like to change the water when the nitrates reach 20. Some people wait till the nitrates are closer to 40 ppm
. It depends on how sensitive the fish are.
I know it's terrible to watch a fish that's sick. My betta lingered like this for about 10 days before he died.
If the betta is about 18 months old, though, he may be getting old. I estimate my betta was about a year and 10 months old when he died. I hope your betta gets better!