You could have a look at LDA025, Pitbull pleco.
This is a really small species.

I don´t think i can get more oxygen into the tank.

No I didn´t open it up as it already had burst.

I think there were some kind of constipation.

I have trouble seeing that this should be trouble with oxygen.
Why do you think that?
The other two L226 are doing just fine.
I think that the one that passed away had some kind of internal problem that caused the "bulb"
Could perhaps be that she didn´t get all the eggs out the first time?

Passed away during the night

I have three L226 that I´ve been in the same tank for about a year and a half. They spawned for the first time a couple of weeks ago but the male didn´t guard the eggs good enough so I didn´t get any fry. The last couple of days one of them has been fast breathing and today I saw this: http://img2.pu ...