hi Doodle, sorry for your loss. While i am not an expert , i have 2 L128's, one is also 5'', and a smaller one. How long did you have him for ? When you did
your wc,did you remember to keep the water level lower than the rim ? I keep a pc of blue tape on the glass as a marker so I don't overfill. L128's need a lot of oxygen. I have read of someone who lost 4 due to wc and unintentially filled the tank too high. I have 2 HOB's and 2 air stones too.
Yes, they do get larger than 5'', I have seen full grown adults and they get big, wide and fat heads .(.best way i can explain), but they would make my 5 inch phantom look itty bitty.
I could be wrong,but if your water is fine and it showed no other signs of illness, I would lean toward oxygen ,but also ,not knowing your set up it would be hard to say for sure. Sorry again.