Young Flounder: Behavior Change + Diet Question

[simonero]

I installed a small beach for him last night and I think it helped.  When I woke up this morning he was chillin' half-covered in his little sand basin.  It's possible the blackworms I gave him just burrowed into the sand, but I couldn't see them, so I'm hoping he ate them... ! =)
 
I think I may look into trying out some shrimp for him at some point.  I love your description of your flounder MBOU, I want my flounder to act like that when it's time to eat lol

 

[DerpPH]

Keep observing him and look for signs of stress or illness as you are uncertain if he ate or not.

 

[yargnits]

if your concerned about it eating then chop up some earthworms (think the americans call them nightcrawlers)  and put them in over night then fish out any leftovers in the morning.
if you have chance i would put in whole earthworms as they last for several days in water and the fish can feed when it wants ( I would expect flounder to feed at night ) 
 
for reference i kept a flounder for a while but it died as a resullt of injury from one of my rays. i fed it earthworms until then though . 
 
hope it helps. flounder are really nice. 
 
some pics when you have chance  

 

[simonero]

Keep observing him and look for signs of stress or illness as you are uncertain if he ate or not.

 
What are considered signs of stress or illness?  I'm new to this... I don't know how to tell with fish, aside from something physical that might be obvious.
 
I gave him a beach last Monday and he literally didn't leave once until a couple hours ago, not even at night.  I put in two tiny bumblebee gobies (coincides with him leaving the beach), which he seems to be getting along with.  He's been out in the open since shortly after I put them in.  I know he's a sand-lover, but I thought it was a little odd that he basically didn't move.  I like to think he was just lonely but I'm aware that's most likely projected from my own fondness of the lil guy lol.
 
Another thing to note - I brought my water in to be checked a day after the substrate change.  I was told that my pH and ammonia were perfect (stayed 7 and 0 ammonia) but that I had .75 nitrite - he said this was high but a normal part of cycling and would be fine in a few days.  LFS lie, or possible reason for sand behavior change?