I'd chalk it up to his genetics too...
Diet is mostly chopped mussel and prawns, with some good-quality pellet food as supplements.
Try reversing that.Totoro is past the stage where he needs mostly protein and needs a wider variety. If the pellets are mostly vegetable-based will help if he's super picky.
Ponyo II is still at a stage where the protein is spurring growth but you'd want to back that down too around March/April (6" at current rate) so probably better to get him use to a variety now too.
upping that to 80F (26.6C).While it doesn't sound like much,3 degrees F may be just enough to speed metabolism where he'll eat better and accept more. No more than that and watch behavior as you do it.
Totoro and Ponyo are fed twice a day. Totoro often rejects his food (he spits it out whole), which can't be helping this
Totoro's at a stage where two feedings may/may not be what he wants or needs.Ponyo's at a stage where IMO it isn't enough.
In all honesty,an O isn't going to starve unless something's up so the two feedings probably work out if Ponyo is thoroughly cleaning up behind Totoro's rejects.
Upping temp slightly,wider variety on the food,and
keeping nitrates low as possible (so test,then set water change routine) is all I've got as you can't "force" growth if it isn't there. The only possible issue might be accelerating Ponyo's growth which would make for more food competition that Totoro can't win...and a chunky,overweight,Ponyo.