Over the weekend, my mom decided to get two african dwarf frogs for her 5 gallon tank that has 3 Neon Tetras and 1 betta, as I know a lot more about aquariums and ect she has me take care of it for her. Anyway, they were doing great for 3 days until this morning, 1 died. It seemed out of nowhere almost, it was eating just fine, moving around, and going up to get air when it normally would, and then sudddenly, he's floating lifeless and slightly bloated. I understand the bloating issue is something that just happens when you buy these guys, as a lot of pet stores get frogs in that already have it, but what happened after is what was weird for me. The second frog was fine earlier as well, 3 hours pass after the last one's death and he's struggling to swim up for air, so I take a closer look and his back right leg is insanely swelled up and has some sort of fungus on it, which seems to have grown insanely fast as it's got very long fibers.
Does anyone know what the heck this is? I'm aware of red leg and bloating, and it doesn't seem to be either of these. I believe the other one is dead because of the same issue. Currently, he's still alive and quarantined as i don't want my mom's fish to possibly be in danger and the small container I have him in, he can reach for air easier. I added some anti ick stuff/water conditioner and i'll update this if it helps anything. Thanks.
Pictures:

Does anyone know what the heck this is? I'm aware of red leg and bloating, and it doesn't seem to be either of these. I believe the other one is dead because of the same issue. Currently, he's still alive and quarantined as i don't want my mom's fish to possibly be in danger and the small container I have him in, he can reach for air easier. I added some anti ick stuff/water conditioner and i'll update this if it helps anything. Thanks.
Pictures:


