Yep ... I am asking for advice again!
I have had a couple of clutches of Peacock Gudgeon eggs, and although they have grown very slowly, I have managed to raise a few babies. Only thing is, quite a few seemed to sucumb to some sort of swim bladder problem. They lost all boyancy, and shuffled around the bottom of the tank. They lasted a long time like that, and I think starvation knocked a few out in the end.
Is this likely to be congenital?
I know bad water sometimes causes this, but I was changing 20% at least almost every day.
Any advice appreciated!

I know with some fry, they have to be able to get to the top of the water to get some air into there bladder. Could that be the problem? maybe they aren't reaching the top to do this.

They may just have more food near the bottom in the tank, and since they haven't realized where the biggest source of food is yet (you) they may just want to stay near the micoorganisms on the bottom. And as for starvation, it's just survival of the fittest, thats the way it usually works :)