I had the 5 gal set up with gravel/plants for about a week, then I addded the betta. He was doing great in there, very happy for about a week and a half, then I added two 1.5-inch cory cats. The corys looked fine and left the betta alone, but the betta was shy of them and would hide. Then after a few days he started loosing color. I thought maybe he would get used to the cory cats, but he didn't so I moved him. This tank is new. For all my tanks, i do 10 - 15% water changes weekly and a 25% change monthly. I have small-sized gravel in both tanks, and power filters. in the 5 gal I have 1 java fern, 1 anubis, and 1 mondo grass. In the 1.5 gal, I just have 1 anubis. The water temp in both tanks is 76 - 78 degrees. like I mentioned, i add a small amount of aqu. salt to my tanks, approximately 1/2 tsp per gal. it doesn not harm the plants at that level but its good for your fish. When I prepare water for changes, I add water conditioner and let it sit in an open container for 24hrs. Other than that, i haven't added anything. This betta is new, I bought him from a pet store less than 1 month ago. As far as I know, he hasn't been sick. he looked great when I bought him, and he looked great until i added the corys, which are also fine, there is nothing wrong with them. He is still eating, but he looks awful with his color and clamped fins., those are the only symptoms. Both these tanks are less than 1 month old.
I had the water in my 5 gal tested at a local pet store, and they said the water is fine. i do not have numbers though. I did not have the water in the 1.5 gal tested, but it should be good too. I keep up with water changes and use conditioner, have live plants, don't over feed. I can post a pic tomorrow.