Just thought I'd post this pic of my Azureus Tadpole. He's been growing at the same rate as other tads from the same clutch. I do daily water changes and it continues to grow but unfortunately... so does the bubble. I'm hoping things change when he pops some legs.
DON'T put it down! There is a chance. I've seen it before, and sometimes it can be fixed.
Start doing daily, 100% water changes. Take out any leaves, and of course any detritus on the bottom. Do not feed. Do NOT feed. Nothing. Zip. Zero. That can be hard to follow through with if you are an empathetic person. Remind yourself that he is already dead if you don't follow through.
When you are absolutely, 100% sure that the bubble is gone, give him another couple of days. Then begin feeding as a reduced rate for a while. Watch him closely for a while. If he balloons again, you need to do the treatment longer.
This treatment was not designed by me. It's tried and true, by many other members. It's not really a "secret", but it's a nugget of knowledge that could stand to be passed around some more.
Explanation: I believe it is a bacterial infection? In any case, I believe the root cause is rotting food in the belly, possibly compounded by water quality. By providing ultra clean conditions, you are just giving him the assistance he needs to be able to fight it off himself. I'm sure there are various other causes for this condition. Perhaps that is why it only works some of the time. I believe it works often enough to give it a try.
Let me see if I can find some references for you.
Might also want to consider a hypertonic solution of amphibian ringer's solution, or simply adding a bit of aquarium salt to the water until you reach 6 PPT.
This treatment doesn't seem to get mentioned much, but also works well for fungus on tads if it's caught very early.
Here is how I treated a tad for bloat with instant ocean salt solution. I made up 1 liter of solution at 6 PPT in RO water. One level teaspoon is very close to 6 grams of dry salt. I started with a 2 PPT solution and increased to 6 PPT by the end of day 1. There was notable improvement after two days, and by the end of day 3 it was very active and basically back to normal. On the following day I gradually reduced the salinity again back to regular RO water. Added a bit of tad tea, aged almond leaf and resumed feeding. Tad seems to be doing well. I'll update when it morphs.
I'm sure it's quite possible, simply switching to pure RO or spring water and stopping all feeding, as others have recommended would work as well.