I cut mine normally in a circle, freeze them and pull out as I need them.
Some people boil them for a minute or two.
I use a fork, pierce them and lay the fork in the bottom of the tank so the cucumber stands up and they will get it. They like squash as well.
Well, He won't eat the veggies, I have the same guy (Charlie) I'd say this is not normal. I'd try a 30% water change and see how he likes that, it just seems like a problem with the water parameters to me.
I keep three Dojo's in a 125 and they are everywhere, I can't see them doing real well in a 50 gallon. Don't seem like enough swimming space.
I'm no expert, I could be wrong, It seems I am more than not lately.