Gas bubbles, help?

Hi all. I have 13 salamander larvae that are around 1 month old. I have no clue what kind they are as I collected them as eggs from my mom's pond thinking they were frog eggs.

Here's the problem. I had 3 toad tadpoles with my salamanders. I had read that during metamorphosis, toads secret a toxic substance which prevents other tadpoles from forming. I didn't fact check. I decided it was time to clean the tank anyways, and what harm could happen? I took my larvae out of the tank. I did a 50% water change and ran my fish filter for about an hour. I'd done this once before with no adverse effects. While waiting for a chance to go to the pond for more daphnia, I fed my larvae frozen bloodworms for 3 days. Most of them seemed to be doing fine. 3 days later two of my larvae were belly up.

It's been a week now and they're still alive, but still belly up. I had them in isolation for 3 days at first, and when they were swimming belly down I re-introduced them back to the tank. They went belly up again overnight so I took them back out for isolation. After 4 days they were doing fine again so I re-introduced them last night. They were belly up this morning with huge gas bubbles, so I took them out again.

I'm worried about my little babies. Everyone else is doing great, but these two guys are struggling to stay alive. I've been trying to feed them frozen bloodworms and daphnia, but they don't seem to eat much. Does anyone have any advice?

Put them in a separate tank and make sure the water parameters are normal. Hopefully the problem will right itself.