From my personal experience, African Dwarf Frogs make really good tank mates for Betta fish - but the tank needs to be sizable (at least a five gallons with filtration). And as they are really slow at finding food, I found that over feeding blood worms and brine shrimp worked really well. The food my crown tail (his name is Nova) wouldn't eat would sink to the bottom and my two frogs would find it and eat it. Be careful though, when introducing them. I once had an ADF (NOT an African Clawed, it was definitely a dwarf) who killed two betta fish and ate one. I thought they were dying because they were sick or something, but I actually saw the frog attacking Nova one day. So I bought a new tank and separated them and Hannibal (the frog) lived the rest of his life alone.
Wow. Thats a little scary.
Well, I made sure the ones that I got weren't African Clawed frogs first.
Now that I know they are AFDs, I've been supervising them constantly. They seem pretty good right now. I had three, but had to take one back because it was pretty aggresive. Now that I've read your story, I'm glad that I did take it back!
Initially, I bought three; two lived with my daughter's goldfish and one lived with my Betta. I left for two weeks and had my room mate take care of it, when I got home it looked really ragged like it had fin rot or something, the next day it was dead and looked like it was "decaying" (turns out that Hannibal was eating it). So I made my room mate buy me a new fish, and it died within about three days. I still didn't understand, I never really saw the fish and frog interact - the frog always seemed to be hiding. So I bought Nova, and then I noticed.
It freaked me RIGHT out. The other two were perfectly fine with my daughters goldfish and eventually, in with Nova. But Hannibal lived in a 3 gallon alone, until the day he died. He always sang, it was a creepy noise.
I'm sorry about your other two fish, thats terrible. I'm glad you realized the cause though.
I am still keeping a close eye on the frogs. So far so good. Skittles has one with him, and he LOVES the little guy. He follows it around, and swims next to it. They sit in their little cave together. Everythings good with that little guy.
The one with my girl Ocean, seems a little food-aggresive, but other than that he seems fine. Ocean doesn't even like the frog food, but follows it to the bottom. The frog jumps at her when she does that. I am keeping a close eye on it for now.