Blue Dolphin (Hap) Problem

[doresy]

My 4"-5" Blue Dolphin is spending a lot of time diving sideways into the substrate, against rocks and even the 3D background. It's been doing it for a few weeks now but now seems to be doing it more and more.
 
It shows no outwardly signs of anything untoward and feeds well. The water (as it has for many years) shows perfect levels. No other fish is showing this behaviour.
 
Is it possible it's frustrated or outgrowing my 4 foot tank?
 
 

 

[techen]

If it's "grinding" against objects there could be something in the water bugging them or white spot. Worth checking.

 

[doresy]

Had a good look at it today. There are absolutely no signs of white spot or anything else for that matter other than a couple of marks where the fish has battered itself against the rocks.
 
Some of the time he acts ok then he sideways flicks against the gravel then hits the rocks quite violently. Not sure if it's connected but occasionally it swims backwards in short bursts.
 
I repeat that all this behaviour has been going on for several weeks and no other fish is acting up