I'm not really sure, but I think our male betta got this disease. I'm going to post the long story, because I'm not sure what is relevant information and what is not. We have a 30-ish gallon tank with a filter and an aerator. We have 10 goldfish, two small corydoras, two plecos, and now one betta fish

. We also had one silver/grey fish. It had long flowing fins and about three dark stripes on her body. (I honestly have no idea what breed it was, we got it from my step mom because she was sick of her fish tank because no one looked at the fish and she was just going to put it, the corydoras, and a pleco in an outside pond, so we rescued them and started our aquarium.) She had mentioned that it was very old, at least 6 or 7 years, and probably wouldn't live long. So last week, it started moving kind of strangely, like it's pectoral fins weren't working right. We put her in a separate bowl and within about a day, her swim bladder obviously wasn't working because she couldn't swim upright or come off the bottom of the bowl. She died about 12 hours later. We weren't really that surprised because, like I said, she was old, and none of the other fish showed any symptoms.
Then about 3 days ago, our female betta started hiding behind the filter. This was very out of character, because she normally hangs out at the top of the tank and likes to look at people. Every time we would go to clean the tank she would swim right into the net or if we had to move one of the decorations, she would rub against our hands. (I know, she's a little strange on a good day, but it was really weird that she was hiding when normally she was so friendly.) None of the other fish seemed to be bothering her, and we hadn't had any problems with aggression from any of them. They had all lived together just fine for about 3 months now. We checked the pH and everything seemed normal, so we just decided to watch and see. Well, the next day, the male betta started hiding too, so we checked the pH again and it was a little acidic so we got a buffer tablet yesterday. Within a couple hours, the female betta seemed to be doing better, but the male was still acting weird. This morning we found him hiding in between one of the decorations and the aerator. My fiance used the net to move the decoration a little and make sure he was still alive (he wasn't moving) and he was, but he was laying on his side and only moved a little. The female betta saw him, went to the bottom and just laid next to him for about two minutes. It was really sad. I had never really considered the idea that fish have that complex of emotions. We took him out and put him in a separate bowl. We put in some Lifeguard All-In-One. An hour later, he was moving more but still on his side. I went to get some aquarium salt to try that, but by the time I got back, he was already gone. The female betta seems to be acting pretty much normal now--well, her version of normal--but she was still looking out the tank like she was looking for him

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Other info:
We live in Iowa, so I'm not sure what the likelihood is that our bettas came from any of the same breeders as the others that have been affected. The male came from a local pet store (I doubt if they breed them there) and the female came from Wal-Mart. We've had them for at least 3 months with no problems until now and have not switched foods or anything. Also, the female doesn't show any signs of a bacteria or decomposition on her fins or body. The male didn't either until this morning, it looked like he had a few bloody streaks on his body (kind of hard to tell, he was a darkish red). His gills definitely seemed swollen before I went to the store. Now his body seems to have some white spots on it that weren't there before.
So I guess my questions are:
Has anyone heard anymore about this? If so, what have you found out?
Can it be transmitted between species?
How likely is it that the female is going to get sick now too?
What should I do with the male's body?
I started cleaning out the entire tank. I'm planning on sterilizing all the decorations and gravel and washing the tank itself with diluted bleach. Is there anything else I can do to make sure the other fish are safe? Should I treat them with some of the All-In-One or a different medicine as a precaution or put some aquarium salt in with the new water that I fill the tank with when it's clean?
Any advice would be much appreciated. Thanks.